In reply to@lassulus:lassul.us
That creates an image file with a disko defined partition layout and some extra files you pass to it

 nix build .#nixosConfigurations.pi.config.system.build.image
warning: Git tree '/home/matthew/git/nix.how/clients/machines' is dirty
trace: warning: system.stateVersion is not set, defaulting to 24.05. Read why this matters on https://nixos.org/manual/nixos/stable/options.html#opt-system.stateVersion.
error: builder for '/nix/store/7mlf74xlxwhy4jq4rvbnjazpx5kd8bpl-image.drv' failed with exit code 1;
       last 7 log lines:
       > structuredAttrs is enabled
       > Running phase: patchPhase
       > 'repart.d' -> '/build/amended-repart.d'
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: buildPhase
       > Building image with systemd-repart...
       > unshare: unshare failed: Invalid argument
       For full logs, run 'nix log /nix/store/7mlf74xlxwhy4jq4rvbnjazpx5kd8bpl-image.drv'.

The way I see it there are only two problems remaining with disko:

1. no compression for diskImages output
2. no hostPlatform recognition (build on hostPlatform vm and run binfmt inside of that accelerated kvm machine)

The way I see it there are only two problems remaining with disko:

1. no compression for diskImages output
2. no hostPlatform recognition (build on hostPlatform vm and run binfmt for arm64 inside of that accelerated kvm machine)

corpo-disko-images> + findmnt /dev/disk/by-partlabel/disk-disk1-firmware /mnt/firmware
corpo-disko-images> + mount /dev/disk/by-partlabel/disk-disk1-firmware /mnt/firmware -t vfat -o defaults -o X-mount.mkdir
corpo-disko-images> + /nix/store/gc39rhvazg28s7y80c4c0a24x3hsv92p-postMountHook.sh
corpo-disko-images> + rm -rf /tmp/tmp.GkPSSeFxnb
corpo-disko-images> warning: the group 'nixbld' specified in 'build-users-group' does not exist
corpo-disko-images> cp: cannot access '/nix/store/fikh9lqcr2vfz3ncq3sihqzhjw482yps-mbrola-voices-3.3/data/gr1': Cannot allocate memory
corpo-disko-images> cp: cannot access '/nix/store/nw17gpncwy29l0r8kaxzss6nb4c1bh6s-linux-firmware-20240513-zstd/lib/firmware/qcom/sm8250': Cannot allocate memory
corpo-disko-images> [  122.745756] reboot: Power down

In reply to @lassulus:lassul.us
https://github.com/nix-community/disko/blob/master/lib/make-disk-image.nix#L6

This is pretty close to working

{ pkgs, ... }:
let
  configTxt = pkgs.writeText "config.txt" ''
    [pi4]
    kernel=u-boot-rpi4.bin
    enable_gic=1

    # Otherwise the resolution will be weird in most cases, compared to
    # what the pi3 firmware does by default.
    disable_overscan=1

    # Supported in newer board revisions
    arm_boost=1

    [cm4]
    # Enable host mode on the 2711 built-in XHCI USB controller.
    # This line should be removed if the legacy DWC2 controller is required
    # (e.g. for USB device mode) or if USB support is not required.
    otg_mode=1

    [all]
    # Boot in 64-bit mode.
    arm_64bit=1

    # U-Boot needs this to work, regardless of whether UART is actually used or not.
    # Look in arch/arm/mach-bcm283x/Kconfig in the U-Boot tree to see if this is still
    # a requirement in the future.
    enable_uart=1

    # Prevent the firmware from smashing the framebuffer setup done by the mainline kernel
    # when attempting to show low-voltage or overtemperature warnings.
    avoid_warnings=1
  '';
in
{
  disko.devices = {
    disk = {
      disk1 = {
        imageSize = "10G";
        type = "disk";
        device = "/dev/mmcblk0";
              postCreateHook = ''
                lsblk
                sgdisk -A 2:set:2 /dev/vdb
              '';
        content = {
          type = "gpt";
          partitions = {
            firmware = {
              size = "30M";
              type = "EF00";
              content = {
                type = "filesystem";
                format = "vfat";
                mountpoint = "/firmware";
                postMountHook = toString (pkgs.writeScript "postMountHook.sh" ''
                  (cd ${pkgs.raspberrypifw}/share/raspberrypi/boot && cp bootcode.bin fixup*.dat start*.elf *.dtb /mnt/firmware/)
                  cp ${pkgs.ubootRaspberryPi4_64bit}/u-boot.bin /mnt/firmware/u-boot-rpi4.bin
                  cp ${configTxt} /mnt/firmware/config.txt
                '');
              };
            };
            root = {
              name = "root";
              size = "100%";
              content = {
                type = "filesystem";
                format = "bcachefs";
                mountpoint = "/";
              };
            };
          };
        };
      };
    };
  };
}

This is pretty close to working

{ pkgs, ... }:
let
  configTxt = pkgs.writeText "config.txt" ''
    [pi4]
    kernel=u-boot-rpi4.bin
    enable_gic=1

    # Otherwise the resolution will be weird in most cases, compared to
    # what the pi3 firmware does by default.
    disable_overscan=1

    # Supported in newer board revisions
    arm_boost=1

    [cm4]
    # Enable host mode on the 2711 built-in XHCI USB controller.
    # This line should be removed if the legacy DWC2 controller is required
    # (e.g. for USB device mode) or if USB support is not required.
    otg_mode=1

    [all]
    # Boot in 64-bit mode.
    arm_64bit=1

    # U-Boot needs this to work, regardless of whether UART is actually used or not.
    # Look in arch/arm/mach-bcm283x/Kconfig in the U-Boot tree to see if this is still
    # a requirement in the future.
    enable_uart=1

    # Prevent the firmware from smashing the framebuffer setup done by the mainline kernel
    # when attempting to show low-voltage or overtemperature warnings.
    avoid_warnings=1
  '';
in
{
  disko.devices = {
    disk = {
      disk1 = {
        imageSize = "10G";
        type = "disk";
        device = "/dev/mmcblk0";
              postCreateHook = ''
                lsblk
                sgdisk -A 2:set:2 /dev/vdb
              '';
        content = {
          type = "gpt";
          partitions = {
            firmware = {
              size = "30M";
              type = "EF00";
              content = {
                type = "filesystem";
                format = "vfat";
                mountpoint = "/firmware";
                postMountHook = toString (pkgs.writeScript "postMountHook.sh" ''
                  (cd ${pkgs.raspberrypifw}/share/raspberrypi/boot && cp bootcode.bin fixup*.dat start*.elf *.dtb /mnt/firmware/)
                  cp ${pkgs.ubootRaspberryPi4_64bit}/u-boot.bin /mnt/firmware/u-boot-rpi4.bin
                  cp ${configTxt} /mnt/firmware/config.txt
                '');
              };
            };
            root = {
              name = "root";
              size = "100%";
              content = {
                type = "filesystem";
                format = "bcachefs";
                mountpoint = "/";
              };
            };
          };
        };
      };
    };
  };
}