How does one put files into the image built for Google Cloud Engine?

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I’m trying to prebuild an image for google cloud with certain things installed and certain files present (custom packages, configuration, public keys, …). Building the image goes fine and using a separate configuration.nix for the guest works, but once I unpack the raw.tar.gz and mount it, only the configuration.nix is present but none of the packages therein nor custom files.

This is what I use to build the image

#/usr/bin/env bash

# From https://github.com/NixOS/nixpkgs/blob/5477e09999685a1468b65caea46b558e2245ee5d/nixos/maintainers/scripts/gce/create-gce.sh
nix-build '<nixpkgs/nixos/lib/eval-config.nix>' \
   -A config.system.build.googleComputeImage \
   --arg modules "[ (import /home/michael/tmp/nixos-gce/configuration.nix {})  <nixpkgs/nixos/modules/virtualisation/google-compute-image.nix> ]" \
   --argstr system x86_64-linux \
   -o gce \
   -j 10

# Mount the raw in the TAR with
# https://www.baeldung.com/linux/img-raw-image-dump-file-management
# fdisk --list disk.raw
# START_PARTION=$(( START_PARTITION*SECTOR_SIZE ))
# sudo losetup --offset $START_PARTITION /dev/loop3 disk.raw
# mkdir target
# sudo mount /dev/loop3 target

This is the host configuration.nix to point to a custom guest-configuration.nix

{ ... }:
{
  imports = [
    <nixpkgs/nixos/modules/virtualisation/google-compute-image.nix>
  ];

  virtualisation.googleComputeImage.configFile = "${/home/michael/tmp/nixos-gce/guest-configuration.nix}";
}

and the guest-configuration.nix (simplified)

{ pkgs, lib, ... }:
{
  imports = [
    <nixpkgs/nixos/modules/virtualisation/google-compute-config.nix>
    <nixpkgs/nixos/modules/virtualisation/google-compute-image.nix>
    lanzaboote.nixosModules.lanzaboote
  ];

  # This file should end up in the image
  environment.etc."test-file.txt" = {
    enable = true;
    source = ./this-test.txt;
  };

  # /home/user should be created and exist on the image
  users.groups.user = {};
  users.users.user = {
    isNormalUser = true;
    createHome = true;
    home = "/home/user";
    extraGroups = [
      "google-sudoers"
    ];
    authorizedKeys.keys = [
      "ssh-ed25519 CUSTOM_KEY_HERE"
    ];
  };
}