nixos-config/lib/gen-iso.nix

lib: system:

let
  sys = lib.nixosSystem {
    inherit system;
    modules = [
      (
        { modulesPath, ... }:
        {
          imports = [
            "${modulesPath}/installer/cd-dvd/installation-cd-minimal.nix"
          ];
          isoImage.squashfsCompression = "gzip -Xcompression-level 1";
        }
      )
    ];
  };

in
sys.config.system.build.isoImage