nixos-rebuild-ng: implement build-image

[thiagokokada]

Port of #347275 to nixos-rebuild-ng.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

Add a 👍 reaction to pull requests you find important.

[thiagokokada]

CC @phaer.

[phaer]

Very nice! Was planning to implement this next week and there it is - with tests and all, thank you! 🎉

Code LGTM - but i haven't run anything yet, but can do so tomorrow or on Tuesday.

(build-image will become much more useful once #359345 is merged in case you or other readers have time for a review)

[thiagokokada]

Very nice! Was planning to implement this next week and there it is - with tests and all, thank you! 🎉

Code LGTM - but i haven't run anything yet, but can do so tomorrow or on Tuesday.

(build-image will become much more useful once #359345 is merged in case you or other readers have time for a review)

I would appreciate the testing, especially if you can test the non-Flakes code path since I have no systems to test it.

[phaer]

Successfully built images with this :)

I would appreciate the testing, especially if you can test the non-Flakes code path since I have no systems to test it.

For a simple test of the CLI, I think it's enough do to that with a minimal nixos instance? i.e. with file.nix being

{
  pkgs ? import <nixpkgs> { },
}:
pkgs.nixos {
  system.stateVersion = "25.05";
}

You could test the --file param with i.e.:
nixos-rebuild-ng build-image --image-variant lxc --file file.nix

And given an attrs.nix defined as

{
  host = import ./file.nix { };
}

You could then do nixos-rebuild-ng build-image --image-variant lxc --file attrs.nix --attr host.