buildLayeredImage: Allow empty store, no paths to add

This is useful when buildLayeredImage is called in a generic way
that should allow simple (base) images to be built, which may not
reference any store paths.

pkgs/build-support/docker/examples.nix

@@ -258,4 +258,26 @@ rec {
     maxLayers = 2;
   };
 
+  # 17. Create a "layered" image without nix store layers. This is not
+  # recommended, but can be useful for base images in rare cases.
+  no-store-paths = pkgs.dockerTools.buildLayeredImage {
+    name = "no-store-paths";
+    tag = "latest";
+    extraCommands = ''
+      chmod a+w bin
+
+      # This removes sharing of busybox and is not recommended. We do this
+      # to make the example suitable as a test case with working binaries.
+      cp -r ${pkgs.pkgsStatic.busybox}/* .
+    '';
+    contents = [
+      # This layer has no dependencies and its symlinks will be dereferenced
+      # when creating the customization layer.
+      (pkgs.runCommand "layer-to-flatten" {} ''
+        mkdir -p $out/bin
+        ln -s /bin/true $out/bin/custom-true
+      ''
+      )
+    ];
+  };
 }