feat: add generate_empty_layers attribute to js_image_layer

e2e/js_image_docker/BUILD.bazel

@@ -42,6 +42,7 @@ js_binary(
 js_image_layer(
     name = "layers",
     binary = ":bin",
+    generate_empty_layers = True,
     root = "/app",
     visibility = ["//visibility:__pkg__"],
 )
@@ -68,6 +69,17 @@ container_layer(
     tars = [":package_store_3p_tar"],
 )
 
+filegroup(
+    name = "package_store_1p_tar",
+    srcs = [":layers"],
+    output_group = "package_store_1p",
+)
+
+container_layer(
+    name = "package_store_1p_layer",
+    tars = [":package_store_1p_tar"],
+)
+
 filegroup(
     name = "node_modules_tar",
     srcs = [":layers"],
@@ -101,6 +113,7 @@ container_image(
     layers = [
         ":node_layer",
         ":package_store_3p_layer",
+        ":package_store_1p_layer",
         ":node_modules_layer",
         ":app_layer",
     ],

js/private/js_image_layer.bzl

@@ -279,7 +279,7 @@ def _runfile_path(ctx, file, runfiles_dir):
     return paths.join(runfiles_dir, to_rlocation_path(ctx, file))
 
 def _build_layer(ctx, type, all_entries_json, entries, inputs):
-    if not entries:
+    if not entries and not ctx.attr.generate_empty_layers:
         return None
 
     entries_json = ctx.actions.declare_file("{}_{}_entries.json".format(ctx.label.name, type))
@@ -490,6 +490,9 @@ js_image_layer_lib = struct(
         "platform": attr.label(
             doc = "Platform to transition.",
         ),
+        "generate_empty_layers": attr.bool(
+            doc = "Generate layers even if they are empty.",
+        ),
     },
 )