feat: add an owner arg to py_image_layer (#545)

I needed to be able to change the owner of some files in the final oci
image. `mtree_mutate` seemed the right way to do it, but I couldn't
control the mtree while using `py_image_layer` , so this adds an `owner`
option to handle the `mtree_mutate`

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes
- Breaking change (forces users to change their own code or config): no
- Suggested release notes appear below: no

### Test plan

<!-- Delete any which do not apply -->

I tested by adding an `owner` attribute to an image I was generating.
I did not update any tests, but if this change is approved, I can go
back and add if we think its necessary.

Author: vinnybod

docs/py_image_layer.md

@@ -30,7 +30,7 @@ oci_image(
 ```
 """
 
-load("@aspect_bazel_lib//lib:tar.bzl", "mtree_spec", "tar")
+load("@aspect_bazel_lib//lib:tar.bzl", "mtree_mutate", "mtree_spec", "tar")
 load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_filegroup")
 
 default_layer_groups = {
@@ -90,7 +90,17 @@ awk < $< 'BEGIN {
         **kwargs
     )
 
-def py_image_layer(name, binary, root = "/", layer_groups = {}, compress = "gzip", tar_args = [], compute_unused_inputs = 1, platform = None, **kwargs):
+def py_image_layer(
+        name,
+        binary,
+        root = "/",
+        layer_groups = {},
+        compress = "gzip",
+        tar_args = [],
+        compute_unused_inputs = 1,
+        platform = None,
+        owner = None,
+        **kwargs):
     """Produce a separate tar output for each layer of a python app
 
     > Requires `awk` to be installed on the host machine/rbe runner.
@@ -119,6 +129,7 @@ def py_image_layer(name, binary, root = "/", layer_groups = {}, compress = "gzip
         compress: Compression algorithm to use. Default is gzip. See: https://github.com/bazel-contrib/bazel-lib/blob/main/docs/tar.md#tar_rule-compress
         compute_unused_inputs: Whether to compute unused inputs. Default is 1. See: https://github.com/bazel-contrib/bazel-lib/blob/main/docs/tar.md#tar_rule-compute_unused_inputs
         platform: The platform to use for the transition. Default is None. See: https://github.com/bazel-contrib/bazel-lib/blob/main/docs/transitions.md#platform_transition_binary-target_platform
+        owner: An owner uid for the uncompressed files. See mtree_mutate: https://github.com/bazel-contrib/bazel-lib/blob/main/docs/tar.md#mutating-the-tar-contents
         tar_args: Additional arguments to pass to the tar rule. Default is `[]`. See: https://github.com/bazel-contrib/bazel-lib/blob/main/docs/tar.md#tar_rule-args
         **kwargs: attribute that apply to all targets expanded by the macro
 
@@ -130,11 +141,24 @@ def py_image_layer(name, binary, root = "/", layer_groups = {}, compress = "gzip
 
     # Produce the manifest for a tar file of our py_binary, but don't tar it up yet, so we can split
     # into fine-grained layers for better pull, push and remote cache performance.
-    mtree_spec(
-        name = name + ".manifest",
-        srcs = [binary],
-        **kwargs
-    )
+    manifest_name = name + ".manifest"
+    if owner:
+        mtree_spec(
+            name = manifest_name + ".preprocessed",
+            srcs = [binary],
+            **kwargs
+        )
+        mtree_mutate(
+            name = manifest_name,
+            mtree = name + ".manifest.preprocessed",
+            owner = owner,
+        )
+    else:
+        mtree_spec(
+            name = manifest_name,
+            srcs = [binary],
+            **kwargs
+        )
 
     groups = dict(**layer_groups)
     groups = dict(groups, **default_layer_groups)