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

e2e/pull/wksp/BUILD.bazel

@@ -1,4 +1,6 @@
 load("@rules_oci//oci:defs.bzl", "oci_image", "oci_push")
+load("@aspect_bazel_lib//lib:diff_test.bzl", "diff_test")
+load("@bazel_skylib//rules:write_file.bzl", "write_file")
 
 oci_image(
     name = "empty",
@@ -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",
+)

e2e/pull/wksp/MODULE.bazel

@@ -2,6 +2,8 @@
 
 bazel_dep(name = "rules_oci", version = "0.0.0", dev_dependency = True)
 bazel_dep(name = "platforms", version = "0.0.7")
+bazel_dep(name = "bazel_skylib", version = "1.5.0")
+bazel_dep(name = "aspect_bazel_lib", version = "2.1.0")
 
 local_path_override(
     module_name = "rules_oci",

oci/private/pull.bzl

@@ -303,6 +303,12 @@ oci_pull = repository_rule(
 )
 
 _MULTI_PLATFORM_IMAGE_ALIAS_TMPL = """\
+filegroup(
+    name = "digest",
+    srcs = ["digest.txt"],
+    visibility = ["//visibility:public"],
+)
+
 alias(
     name = "{target_name}",
     actual = select(
@@ -314,6 +320,12 @@ alias(
 """
 
 _SINGLE_PLATFORM_IMAGE_ALIAS_TMPL = """\
+filegroup(
+    name = "digest",
+    srcs = ["digest.txt"],
+    visibility = ["//visibility:public"],
+)
+
 alias(
     name = "{target_name}",
     actual = "@{original}//:{original}",
@@ -333,6 +345,7 @@ 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"]: