feat: support dockerTools.streamLayeredImage

Support the more optimized way of building docker images.

Uses `isExe` to differentiate between a prebuilt image or a command that spits out an image. See [sources][].

[sources]: https://github.com/NixOS/nixpkgs/blob/0dbb76220d1090753ba7919353c13b96d5969aff/pkgs/build-support/docker/default.nix#L1042-L1044

docs/content/examples/image/image.nix

@@ -1,5 +1,5 @@
 { dockerTools, nginx }:
-dockerTools.buildLayeredImage {
+dockerTools.streamLayeredImage {
   name = "nginx";
   contents = [ nginx ];
   extraCommands = ''

lib/docker/default.nix

@@ -2,13 +2,18 @@
 with lib; {
   copyDockerImages = { images, dest, args ? "" }:
     pkgs.writeScript "copy-docker-images.sh" (concatMapStrings
-      (image: ''
-        #!${pkgs.runtimeShell}
+      (image:
+        let
+          prefix = optionalString (image.isExe or false) "${image} | ${pkgs.gzip}/bin/gzip --fast |";
+          src = if image.isExe or false then "/dev/stdin" else ${image};
+        in
+        ''
+          #!${pkgs.runtimeShell}
 
-        set -e
+          set -e
 
-        echo "copying '${image.imageName}:${image.imageTag}' to '${dest}/${image.imageName}:${image.imageTag}'"
-        ${pkgs.skopeo}/bin/skopeo copy ${args} $@ docker-archive:${image} ${dest}/${image.imageName}:${image.imageTag}
-      '')
+          echo "copying '${image.imageName}:${image.imageTag}' to '${dest}/${image.imageName}:${image.imageTag}'"
+          ${prefix} ${pkgs.skopeo}/bin/skopeo copy ${args} $@ docker-archive:${src} ${dest}/${image.imageName}:${image.imageTag}
+        '')
       images);
 }