I got tired of manually building a Debian install with Docker every time I needed a new VM for whatever reason.

Ok, so in detail, what I want to do. Install Debian by only supplying a static IP address during install. There are other requirements for Kubernetes that need to be met, like not have a swap partition and having Docker installed. I want all that done for me without entering anything.

In my environment I network boot my Debian installer with iPXE, in theory this should work with other PXE boot loaders like PXELinux with minor modifications, probably only with how the kernel options passed into the kernel.

We are going to store my preseed file on a web server. Because it's easy to modify and keep track of. That URL will be http://pxe.example.com/debian10/preseed/docker.cfg. The argument to set that is url. So, we'll add url=http://pxe.example.com/debian10/preseed/docker.cfg to the list.

A little note, because we are pulling in the preseed file through http, the networking must be configured before it can read the preseed file. Because of that we need to set the network config through kernel arguments.

My example will ask for the IP address and default the hostname to a name other than debian. I'll explain that in a section below, that solution I found that through a happy accident.

Kernel options

First set of prompts, the locale information. We will set the locale to en_US because that's where I'm at and what I want. Add debian-installer/locale=en_US to the arguments.

Next is the keyboard configuration. Add keyboard-configuration/xkb-keymap=us.

I need to disable the network auto config because of the static IP. That argument is netcfg/disable_autoconfig set to true. Add netcfg/disable_autoconfig=true.

To set the config to manual, add netcfg/dhcp_options=Configure network manually to our argument list.

I do not want to type in my domain name every time, so we'll add netcfg/get_domain=example.com.

And to set the subnet we'll use netcfg/get_netmask=255.255.255.0.

For the gateway use netcfg/get_gateway=192.168.0.1.

For the DNS nameservers, I have 2 of them. 192.168.0.253 and 192.168.0.254. If you have a single nameserver you do not need quotes around them, if you have multiple surround them with quotes and they are space separated. We will add netcfg/get_nameservers="172.16.40.253 172.16.40.254" to our arguments.

And because I do not want to be prompted for confirmation, add netcfg/confirm_static=true

In my iPXE config file, my final kernel looks like this:

kernel debian10/amd64/linux initrd=initrd.gz vga=788 --- quiet auto \
    url=http://pxe.example.com/debian10/preseed/docker.cfg \
    debian-installer/locale=en_US \
    keyboard-configuration/xkb-keymap=us \
    netcfg/disable_autoconfig=true \
    netcfg/dhcp_options=Configure network manually \
    netcfg/get_domain=veccsolutions.org \
    netcfg/get_netmask=255.255.255.0 \
    netcfg/get_gateway=192.168.0.1 \
    netcfg/get_nameservers="192.168.0.253 192.168.0.254" \
    netcfg/confirm_static=true

Hostname

Next requirement was having the hostname setup. Since I'll probably forget this in the future, I left it to prompt me for it.

To default it to something other than Debian, add the reverse lookup DNS entry to your DNS server. The Debian installer will pick it up and use the hostname from that entry.

Example, set the reverse entry for 192.168.0.10 to docker1.example.com and use the 192.168.0.10 as your static IP. Your hostname during install will default to docker1. I found this by accidentally using the IP of one of my other hosts and it showed that hostname instead of debian. Super cool.

Preseed Configuration

For the preseed file, this gets a little more in depth because of how many options there are. I started with the official preseed file from Debian found here https://www.debian.org/releases/stable/example-preseed.txt

Network Configuration

Just for completeness, I set the same options for the kernel in the preseed file. Not sure if I needed to, but I did anyways. so go through the preseed file and update the values.

The changes for the networking I made where

  • Uncomment d-i netcfg/disable_autoconfig boolean true
  • Uncomment d-i netcfg/get_netmask string 255.255.255.0
  • Uncomment d-i netcfg/get_gateway string 192.168.1.1 and set the value to 192.168.0.1
  • Uncomment d-i netcfg/get_nameservers string 192.168.1.1 and set the value to 192.168.0.253 192.168.0.254. Notice I do not have quotes around it like I did in the kernel parameters.
  • Uncomment d-i netcfg/confirm_static boolean true
  • Comment d-i netcfg/get_domain string unassigned-domain
  • Changed d-i netcfg/get_hostname string unassigned-hostname to example.com

Debian Mirror

I have my own local mirror of the Debian repository, so I made these additional changes. Note, for the installer to pull from the mirror you also need to bring in Debian-updates into the same location as your Debian mirror. My mirror is at http://debmirror.example.com/debian.

  • Uncomment d-i mirror/protocol string ftp and set to http.
  • Change d-i mirror/http/hostname string http.us.debian.org to debmirror.example.com.

Disable Root Login

To increase security, I disabled root login. Make the following change

  • Uncomment d-i passwd/root-login boolean false

User Configuration

I don't want to always set my user up nor do I want to store my password in plain text so I made the additional changes.

  • Uncomment d-i passwd/user-fullname string Debian User and change Debian User to your name
  • Uncomment d-i passwd/username string debian and change debian to your username
  • Uncomment d-i passwd/user-password-crypted password [crypt(3) hash] and change [crypt(3) hash] to the hash that is stored in shadow. Example with password for the password: d-i passwd/user-password-crypted password  $6$/Qd1maqW7desueTZ$UsdZULf7J51luTt/JZZruWhhn2WI349uINXSMksyvP0neFw0G2K1QG0/e81d.F6dXxy0g3mi/ILG6tu0tVQ.Q/

Time zone

Next up is the time zone information. I am in the Mountain time zone so I'll make the following update:

  • Change d-i time/zone string US/Eastern to US/Mountain

Disable NTP

Since these are VM's which get their time from the underlying host, we disable NTP with the following

  • Change d-i clock-setup/ntp boolean true to false

Partition with no swap

The next part was the hardest. Getting the partition mapping to actually work. The most difficult part was the EFI partition. We will make the following changes to the file

  • Uncomment #d-i partman-auto/disk string /dev/sda
  • Change d-i partman-auto/method string lvm to regular
  • Comment d-i partman-auto/choose_recipe select atomic
  • Below the comment block for d-i partman-auto/expert_recipe string I added the following for EFI/UEFI (Generation 2 VM's on Hyper-V).
d-i partman-auto/expert_recipe string                           \
     myboot ::                                                  \
                538 538 1075 free                               \
                    $iflabel{ gpt }                             \
                    $reusemethod{ }                             \
                    method{ efi }                               \
                    format{ }                                   \
                    .                                           \
                900 10000 -1 $default_filesystem                \
                    $lvmok{ }                                   \
                    method{ format }                            \
                    format{ }                                   \
                    use_filesystem{ }                           \
                    $default_filesystem{ }                      \
                    mountpoint{ / }                             \
                    . 
  • For non EFI/UEFI you could try, no guarantees that it will work, I only use EFI/UEFI:
d-i partman-auto/expert_recipe string                           \
     myboot ::                                                  \
                900 10000 -1 $default_filesystem                \
                    $lvmok{ }                                   \
                    method{ format }                            \
                    format{ }                                   \
                    use_filesystem{ }                           \
                    $default_filesystem{ }                      \
                    mountpoint{ / }                             \
                    . 
  • Below that new chunk of text I added d-i partman-auto/choose_recipe select myboot
  • Additionally, add d-i partman-basicfilesystems/no_swap boolean false
  • Also added d-i partman-efi/non_efi_system boolean true. This got rid of a weird error on Hyper-V about an EFI partition not existing.

Package Selection

I only want openssh-server, none of the other stuff. So the following was modified.

  • Uncomment tasksel tasksel/first multiselect standard, web-server, kde-desktop, and removed , web-server, kde-desktop
  • Uncomment d-i pkgsel/include string openssh-server build-essential and removed build-essential
  • Uncomment popularity-contest popularity-contest/participate boolean false

Setup password-less sudo, ssh cert login and install docker

  • Make sure the d-i preseed/late_command string is commented.
  • Right after that commented line add the following, replace [youruser] with your username from above and [public ssh key] the the public ssh key you use to ssh into the server with:
d-i preseed/late_command string \
    echo "[youruser] ALL=(ALL) NOPASSWD: ALL" > /target/etc/sudoers.d/000-[youruser]; \
    in-target mkdir /home/[youruser]/.ssh; \
    in-target /bin/sh -c 'echo "[public ssh key]" > /home/[youruser]/.ssh/authorized_keys'; \
    in-target /bin/sh -c 'chown [youruser]:[youruser] -R /home/[youruser]/.ssh'; \
    in-target /bin/sh -c 'apt install -y apt-transport-https ca-certificates curl gnupg-agent software-properties-common vim'; \
    in-target /bin/sh -c 'curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -'; \
    in-target /bin/sh -c 'add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/debian buster stable"'; \
    in-target /bin/sh -c 'apt-get update'; \
    in-target /bin/sh -c 'apt-get install -y docker-ce docker-ce-cli containerd.io'; \
    in-target /bin/sh -c 'usermod -aG docker [youruser]';

Full preseed file:

#### Contents of the preconfiguration file (for buster)
### Localization
# Preseeding only locale sets language, country and locale.
d-i debian-installer/locale string en_US

# The values can also be preseeded individually for greater flexibility.
#d-i debian-installer/language string en
#d-i debian-installer/country string NL
#d-i debian-installer/locale string en_GB.UTF-8
# Optionally specify additional locales to be generated.
#d-i localechooser/supported-locales multiselect en_US.UTF-8, nl_NL.UTF-8

# Keyboard selection.
d-i keyboard-configuration/xkb-keymap select us
# d-i keyboard-configuration/toggle select No toggling

### Network configuration
# Disable network configuration entirely. This is useful for cdrom
# installations on non-networked devices where the network questions,
# warning and long timeouts are a nuisance.
#d-i netcfg/enable boolean false

# netcfg will choose an interface that has link if possible. This makes it
# skip displaying a list if there is more than one interface.
d-i netcfg/choose_interface select auto

# To pick a particular interface instead:
#d-i netcfg/choose_interface select eth1

# To set a different link detection timeout (default is 3 seconds).
# Values are interpreted as seconds.
#d-i netcfg/link_wait_timeout string 10

# If you have a slow dhcp server and the installer times out waiting for
# it, this might be useful.
#d-i netcfg/dhcp_timeout string 60
#d-i netcfg/dhcpv6_timeout string 60

# If you prefer to configure the network manually, uncomment this line and
# the static network configuration below.
d-i netcfg/disable_autoconfig boolean true

# If you want the preconfiguration file to work on systems both with and
# without a dhcp server, uncomment these lines and the static network
# configuration below.
#d-i netcfg/dhcp_failed note
#d-i netcfg/dhcp_options select Configure network manually

# Static network configuration.
#
# IPv4 example
#d-i netcfg/get_ipaddress string 192.168.1.42
d-i netcfg/get_netmask string 255.255.255.0
d-i netcfg/get_gateway string 192.168.0.1
d-i netcfg/get_nameservers string 192.168.0.253 192.168.0.254
d-i netcfg/confirm_static boolean true
#
# IPv6 example
#d-i netcfg/get_ipaddress string fc00::2
#d-i netcfg/get_netmask string ffff:ffff:ffff:ffff::
#d-i netcfg/get_gateway string fc00::1
#d-i netcfg/get_nameservers string fc00::1
#d-i netcfg/confirm_static boolean true

# Any hostname and domain names assigned from dhcp take precedence over
# values set here. However, setting the values still prevents the questions
# from being shown, even if values come from dhcp.
# d-i netcfg/get_hostname string unassigned-hostname
d-i netcfg/get_domain string example.com

# If you want to force a hostname, regardless of what either the DHCP
# server returns or what the reverse DNS entry for the IP is, uncomment
# and adjust the following line.
#d-i netcfg/hostname string somehost

# Disable that annoying WEP key dialog.
d-i netcfg/wireless_wep string
# The wacky dhcp hostname that some ISPs use as a password of sorts.
#d-i netcfg/dhcp_hostname string radish

# If non-free firmware is needed for the network or other hardware, you can
# configure the installer to always try to load it, without prompting. Or
# change to false to disable asking.
#d-i hw-detect/load_firmware boolean true

### Network console
# Use the following settings if you wish to make use of the network-console
# component for remote installation over SSH. This only makes sense if you
# intend to perform the remainder of the installation manually.
#d-i anna/choose_modules string network-console
#d-i network-console/authorized_keys_url string http://10.0.0.1/openssh-key
#d-i network-console/password password r00tme
#d-i network-console/password-again password r00tme

### Mirror settings
# If you select ftp, the mirror/country string does not need to be set.
d-i mirror/protocol string http
d-i mirror/country string manual
d-i mirror/http/hostname string debmirror.example.com
d-i mirror/http/directory string /debian
d-i mirror/http/proxy string

# Suite to install.
#d-i mirror/suite string testing
# Suite to use for loading installer components (optional).
#d-i mirror/udeb/suite string testing

### Account setup
# Skip creation of a root account (normal user account will be able to
# use sudo).
d-i passwd/root-login boolean false
# Alternatively, to skip creation of a normal user account.
#d-i passwd/make-user boolean false

# Root password, either in clear text
#d-i passwd/root-password password r00tme
#d-i passwd/root-password-again password r00tme
# or encrypted using a crypt(3)  hash.
#d-i passwd/root-password-crypted password [crypt(3) hash]

# To create a normal user account.
d-i passwd/user-fullname string Your Name
d-i passwd/username string user1
# Normal user's password, either in clear text
#d-i passwd/user-password password insecure
#d-i passwd/user-password-again password insecure
# or encrypted using a crypt(3) hash.
d-i passwd/user-password-crypted password $6$/Qd1maqW7desueTZ$UsdZULf7J51luTt/JZZruWhhn2WI349uINXSMksyvP0neFw0G2K1QG0/e81d.F6dXxy0g3mi/ILG6tu0tVQ.Q/
# Create the first user with the specified UID instead of the default.
#d-i passwd/user-uid string 1010

# The user account will be added to some standard initial groups. To
# override that, use this.
#d-i passwd/user-default-groups string audio cdrom video

### Clock and time zone setup
# Controls whether or not the hardware clock is set to UTC.
d-i clock-setup/utc boolean true

# You may set this to any valid setting for $TZ; see the contents of
# /usr/share/zoneinfo/ for valid values.
d-i time/zone string US/Mountain

# Controls whether to use NTP to set the clock during the install
d-i clock-setup/ntp boolean false
# NTP server to use. The default is almost always fine here.
#d-i clock-setup/ntp-server string ntp.example.com

### Partitioning
## Partitioning example
# If the system has free space you can choose to only partition that space.
# This is only honoured if partman-auto/method (below) is not set.
#d-i partman-auto/init_automatically_partition select biggest_free

# Alternatively, you may specify a disk to partition. If the system has only
# one disk the installer will default to using that, but otherwise the device
# name must be given in traditional, non-devfs format (so e.g. /dev/sda
# and not e.g. /dev/discs/disc0/disc).
# For example, to use the first SCSI/SATA hard disk:
d-i partman-auto/disk string /dev/sda
# In addition, you'll need to specify the method to use.
# The presently available methods are:
# - regular: use the usual partition types for your architecture
# - lvm:     use LVM to partition the disk
# - crypto:  use LVM within an encrypted partition
d-i partman-auto/method string regular

# You can define the amount of space that will be used for the LVM volume
# group. It can either be a size with its unit (eg. 20 GB), a percentage of
# free space or the 'max' keyword.
d-i partman-auto-lvm/guided_size string max

# If one of the disks that are going to be automatically partitioned
# contains an old LVM configuration, the user will normally receive a
# warning. This can be preseeded away...
d-i partman-lvm/device_remove_lvm boolean true
# The same applies to pre-existing software RAID array:
d-i partman-md/device_remove_md boolean true
# And the same goes for the confirmation to write the lvm partitions.
d-i partman-lvm/confirm boolean true
d-i partman-lvm/confirm_nooverwrite boolean true

# You can choose one of the three predefined partitioning recipes:
# - atomic: all files in one partition
# - home:   separate /home partition
# - multi:  separate /home, /var, and /tmp partitions
#d-i partman-auto/choose_recipe select atomic

# Or provide a recipe of your own...
# If you have a way to get a recipe file into the d-i environment, you can
# just point at it.
#d-i partman-auto/expert_recipe_file string /hd-media/recipe

# If not, you can put an entire recipe into the preconfiguration file in one
# (logical) line. This example creates a small /boot partition, suitable
# swap, and uses the rest of the space for the root partition:
#d-i partman-auto/expert_recipe string                         \
#      boot-root ::                                            \
#              40 50 100 ext3                                  \
#                      $primary{ } $bootable{ }                \
#                      method{ format } format{ }              \
#                      use_filesystem{ } filesystem{ ext3 }    \
#                      mountpoint{ /boot }                     \
#              .                                               \
#              500 10000 1000000000 ext3                       \
#                      method{ format } format{ }              \
#                      use_filesystem{ } filesystem{ ext3 }    \
#                      mountpoint{ / }                         \
#              .                                               \
#              64 512 300% linux-swap                          \
#                      method{ swap } format{ }                \
#              .
d-i partman-auto/expert_recipe string                           \
     myboot ::                                                  \
                538 538 1075 free                               \
                    $iflabel{ gpt }                             \
                    $reusemethod{ }                             \
                    method{ efi }                               \
                    format{ }                                   \
                    .                                           \
                900 10000 -1 $default_filesystem                \
                    $lvmok{ }                                   \
                    method{ format }                            \
                    format{ }                                   \
                    use_filesystem{ }                           \
                    $default_filesystem{ }                      \
                    mountpoint{ / }                             \
                    .        

d-i partman-auto/choose_recipe select myboot
d-i partman-basicfilesystems/no_swap boolean false
d-i partman-efi/non_efi_system boolean true

# The full recipe format is documented in the file partman-auto-recipe.txt
# included in the 'debian-installer' package or available from D-I source
# repository. This also documents how to specify settings such as file
# system labels, volume group names and which physical devices to include
# in a volume group.

# This makes partman automatically partition without confirmation, provided
# that you told it what to do using one of the methods above.
d-i partman-partitioning/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i partman/confirm_nooverwrite boolean true

# When disk encryption is enabled, skip wiping the partitions beforehand.
#d-i partman-auto-crypto/erase_disks boolean false

## Partitioning using RAID
# The method should be set to "raid".
#d-i partman-auto/method string raid
# Specify the disks to be partitioned. They will all get the same layout,
# so this will only work if the disks are the same size.
#d-i partman-auto/disk string /dev/sda /dev/sdb

# Next you need to specify the physical partitions that will be used. 
#d-i partman-auto/expert_recipe string \
#      multiraid ::                                         \
#              1000 5000 4000 raid                          \
#                      $primary{ } method{ raid }           \
#              .                                            \
#              64 512 300% raid                             \
#                      method{ raid }                       \
#              .                                            \
#              500 10000 1000000000 raid                    \
#                      method{ raid }                       \
#              .

# Last you need to specify how the previously defined partitions will be
# used in the RAID setup. Remember to use the correct partition numbers
# for logical partitions. RAID levels 0, 1, 5, 6 and 10 are supported;
# devices are separated using "#".
# Parameters are:
# <raidtype> <devcount> <sparecount> <fstype> <mountpoint> \
#          <devices> <sparedevices>

#d-i partman-auto-raid/recipe string \
#    1 2 0 ext3 /                    \
#          /dev/sda1#/dev/sdb1       \
#    .                               \
#    1 2 0 swap -                    \
#          /dev/sda5#/dev/sdb5       \
#    .                               \
#    0 2 0 ext3 /home                \
#          /dev/sda6#/dev/sdb6       \
#    .

# For additional information see the file partman-auto-raid-recipe.txt
# included in the 'debian-installer' package or available from D-I source
# repository.

# This makes partman automatically partition without confirmation.
d-i partman-md/confirm boolean true
d-i partman-partitioning/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i partman/confirm_nooverwrite boolean true

## Controlling how partitions are mounted
# The default is to mount by UUID, but you can also choose "traditional" to
# use traditional device names, or "label" to try filesystem labels before
# falling back to UUIDs.
#d-i partman/mount_style select uuid

### Base system installation
# Configure APT to not install recommended packages by default. Use of this
# option can result in an incomplete system and should only be used by very
# experienced users.
#d-i base-installer/install-recommends boolean false

# The kernel image (meta) package to be installed; "none" can be used if no
# kernel is to be installed.
#d-i base-installer/kernel/image string linux-image-686

### Apt setup
# You can choose to install non-free and contrib software.
#d-i apt-setup/non-free boolean true
#d-i apt-setup/contrib boolean true
# Uncomment this if you don't want to use a network mirror.
#d-i apt-setup/use_mirror boolean false
# Select which update services to use; define the mirrors to be used.
# Values shown below are the normal defaults.
#d-i apt-setup/services-select multiselect security, updates
#d-i apt-setup/security_host string security.debian.org

# Additional repositories, local[0-9] available
#d-i apt-setup/local0/repository string \
#       http://local.server/debian stable main
#d-i apt-setup/local0/comment string local server
# Enable deb-src lines
#d-i apt-setup/local0/source boolean true
# URL to the public key of the local repository; you must provide a key or
# apt will complain about the unauthenticated repository and so the
# sources.list line will be left commented out
#d-i apt-setup/local0/key string http://local.server/key

# By default the installer requires that repositories be authenticated
# using a known gpg key. This setting can be used to disable that
# authentication. Warning: Insecure, not recommended.
#d-i debian-installer/allow_unauthenticated boolean true

# Uncomment this to add multiarch configuration for i386
#d-i apt-setup/multiarch string i386


### Package selection
tasksel tasksel/first multiselect standard

# Individual additional packages to install
d-i pkgsel/include string openssh-server

# Whether to upgrade packages after debootstrap.
# Allowed values: none, safe-upgrade, full-upgrade
#d-i pkgsel/upgrade select none

# Some versions of the installer can report back on what software you have
# installed, and what software you use. The default is not to report back,
# but sending reports helps the project determine what software is most
# popular and include it on CDs.
popularity-contest popularity-contest/participate boolean false

### Boot loader installation
# Grub is the default boot loader (for x86). If you want lilo installed
# instead, uncomment this:
#d-i grub-installer/skip boolean true
# To also skip installing lilo, and install no bootloader, uncomment this
# too:
#d-i lilo-installer/skip boolean true


# This is fairly safe to set, it makes grub install automatically to the MBR
# if no other operating system is detected on the machine.
d-i grub-installer/only_debian boolean true

# This one makes grub-installer install to the MBR if it also finds some other
# OS, which is less safe as it might not be able to boot that other OS.
d-i grub-installer/with_other_os boolean true

# Due notably to potential USB sticks, the location of the MBR can not be
# determined safely in general, so this needs to be specified:
#d-i grub-installer/bootdev  string /dev/sda
# To install to the first device (assuming it is not a USB stick):
#d-i grub-installer/bootdev  string default

# Alternatively, if you want to install to a location other than the mbr,
# uncomment and edit these lines:
#d-i grub-installer/only_debian boolean false
#d-i grub-installer/with_other_os boolean false
#d-i grub-installer/bootdev  string (hd0,1)
# To install grub to multiple disks:
#d-i grub-installer/bootdev  string (hd0,1) (hd1,1) (hd2,1)

# Optional password for grub, either in clear text
#d-i grub-installer/password password r00tme
#d-i grub-installer/password-again password r00tme
# or encrypted using an MD5 hash, see grub-md5-crypt(8).
#d-i grub-installer/password-crypted password [MD5 hash]

# Use the following option to add additional boot parameters for the
# installed system (if supported by the bootloader installer).
# Note: options passed to the installer will be added automatically.
#d-i debian-installer/add-kernel-opts string nousb

### Finishing up the installation
# During installations from serial console, the regular virtual consoles
# (VT1-VT6) are normally disabled in /etc/inittab. Uncomment the next
# line to prevent this.
#d-i finish-install/keep-consoles boolean true

# Avoid that last message about the install being complete.
d-i finish-install/reboot_in_progress note

# This will prevent the installer from ejecting the CD during the reboot,
# which is useful in some situations.
#d-i cdrom-detect/eject boolean false

# This is how to make the installer shutdown when finished, but not
# reboot into the installed system.
#d-i debian-installer/exit/halt boolean true
# This will power off the machine instead of just halting it.
#d-i debian-installer/exit/poweroff boolean true

### Preseeding other packages
# Depending on what software you choose to install, or if things go wrong
# during the installation process, it's possible that other questions may
# be asked. You can preseed those too, of course. To get a list of every
# possible question that could be asked during an install, do an
# installation, and then run these commands:
#   debconf-get-selections --installer > file
#   debconf-get-selections >> file


#### Advanced options
### Running custom commands during the installation
# d-i preseeding is inherently not secure. Nothing in the installer checks
# for attempts at buffer overflows or other exploits of the values of a
# preconfiguration file like this one. Only use preconfiguration files from
# trusted locations! To drive that home, and because it's generally useful,
# here's a way to run any shell command you'd like inside the installer,
# automatically.

# This first command is run as early as possible, just after
# preseeding is read.
#d-i preseed/early_command string anna-install some-udeb
# This command is run immediately before the partitioner starts. It may be
# useful to apply dynamic partitioner preseeding that depends on the state
# of the disks (which may not be visible when preseed/early_command runs).
#d-i partman/early_command \
#       string debconf-set partman-auto/disk "$(list-devices disk | head -n1)"
# This command is run just before the install finishes, but when there is
# still a usable /target directory. You can chroot to /target and use it
# directly, or use the apt-install and in-target commands to easily install
# packages and run commands in the target system.
#d-i preseed/late_command string apt-install zsh; in-target chsh -s /bin/zsh
d-i preseed/late_command string \
    echo "user1 ALL=(ALL) NOPASSWD: ALL" > /target/etc/sudoers.d/000-user1; \
    in-target mkdir /home/user1/.ssh; \
    in-target /bin/sh -c 'echo "ssh-ed25519 blahblahblah" > /home/user1/.ssh/authorized_keys'; \
    in-target /bin/sh -c 'chown user1:user1 -R /home/user1/.ssh'; \
    in-target /bin/sh -c 'apt install -y apt-transport-https ca-certificates curl gnupg-agent software-properties-common vim'; \
    in-target /bin/sh -c 'curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -'; \
    in-target /bin/sh -c 'add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/debian buster stable"'; \
    in-target /bin/sh -c 'apt-get update'; \
    in-target /bin/sh -c 'apt-get install -y docker-ce docker-ce-cli containerd.io'; \
    in-target /bin/sh -c 'usermod -aG docker user1';

Conclusion

It took a few hours to figure this all out, but now I can build a fully configured Debian image with Docker with nothing other than putting in an IP address and a DNS entry. As soon as it reboots, I was able to SSH into the system with my private key. It takes about 3 minutes to build a new system from scratch (or reload one if needed). Really nice, I wish I had done this a long time ago.