Create helper function to create NixNG images

Move NixNG images to separate directory

modules/attic.nix

@@ -1,21 +1,4 @@
-{ nixpkgs, pkgs, lib, nixng, config, globals, ... }:
-let
-
-  atticStream = (import ./attic-image.nix {
-    inherit nixpkgs nixng globals;
-    inherit (nixng) nglib;
-  }).config.system.build.ociImage.stream;
-
-  atticImage = pkgs.stdenv.mkDerivation {
-    name = "attic.tar";
-    src = atticStream;
-    dontUnpack = true;
-    buildPhase = ''
-      $src > $out
-    '';
-  };
-in
-{
+{ self, utils, lib, config, globals, ... }: {
   options.attic.enable = lib.mkEnableOption "attic";
 
   config = lib.mkIf config.attic.enable {
@@ -52,7 +35,7 @@ in
 
             spec = {
               containers.attic = {
-                image = "nix:0${atticImage}";
+                image = utils.nixSnapshotterRef (utils.mkNixNGImage "attic" "${self}/images/attic.nix");
                 ports.web.containerPort = 8080;
 
                 env = {