add dockerImage utility

default.nix

@@ -9,7 +9,8 @@ let
 
   self = {
     # nix tools
-    dhallToNix = import ./dhallToNix.nix;
+    dhallToNix = import ./pkgs/dhallToNix.nix;
+    dockerImage = callPackage ./pkgs/dockerImage.nix { };
 
     # other
     MoonPlus = callPackage ./pkgs/moonplus { };

pkgs/dockerImage.nix

@@ -0,0 +1,8 @@
+{ sources ? import ./nix/sources.nix, pkgs ? import sources.nixpkgs { } }:
+
+attrs:
+pkgs.dockerTools.buildLayeredImage {
+  config = {
+    WorkingDir = "/";
+  };
+} // attrs