feat: expose the digest as a filegroup (#542)

Author: aignas

e2e/assertion/wksp/BUILD.bazel

@@ -1,3 +1,5 @@
+load("@aspect_bazel_lib//lib:diff_test.bzl", "diff_test")
+load("@bazel_skylib//rules:write_file.bzl", "write_file")
 load("@rules_oci//oci:defs.bzl", "oci_image", "oci_push")
 
 oci_image(
@@ -12,3 +14,17 @@ oci_push(
     remote_tags = ["latest"],
     repository = "localhost/empty_image",
 )
+
+write_file(
+    name = "want_digest_contents",
+    out = "want_image_digest.json",
+    content = [
+        "sha256:2d4595bbc0fabeb1489b1071f56c26f44a2f495afaa9386ad7d24e7b3d8dfd3e",
+    ],
+)
+
+diff_test(
+    name = "check_they_match",
+    file1 = "want_digest_contents",
+    file2 = "@empty_image//:digest",
+)

oci/private/pull.bzl

@@ -309,6 +309,12 @@ oci_pull = repository_rule(
 )
 
 _MULTI_PLATFORM_IMAGE_ALIAS_TMPL = """\
+filegroup(
+    name = "digest",
+    srcs = ["digest.txt"],
+    visibility = ["//visibility:public"],
+)
+
 alias(
     name = "{target_name}",
     actual = select(
@@ -320,6 +326,12 @@ alias(
 """
 
 _SINGLE_PLATFORM_IMAGE_ALIAS_TMPL = """\
+filegroup(
+    name = "digest",
+    srcs = ["digest.txt"],
+    visibility = ["//visibility:public"],
+)
+
 alias(
     name = "{target_name}",
     actual = "@{original}//:{original}",
@@ -339,6 +351,7 @@ def _oci_alias_impl(rctx):
     available_platforms = []
 
     manifest, _, digest = downloader.download_manifest(rctx.attr.identifier, "mf.json")
+    rctx.file("digest.txt", digest)
 
     if rctx.attr.platforms:
         if manifest["mediaType"] in _SUPPORTED_MEDIA_TYPES["index"]: