feat: provide the sha256 digest as an oci_image target

Fixes #60

WORKSPACE

@@ -64,6 +64,11 @@ load("@aspect_rules_js//js:repositories.bzl", "rules_js_dependencies")
 
 rules_js_dependencies()
 
+load("@aspect_bazel_lib//lib:repositories.bzl", "aspect_bazel_lib_dependencies")
+
+# Workaround for Bazel 5 support
+aspect_bazel_lib_dependencies(override_local_config_platform = True)
+
 load("@rules_nodejs//nodejs:repositories.bzl", "DEFAULT_NODE_VERSION", "nodejs_register_toolchains")
 
 nodejs_register_toolchains(

docs/image.md

@@ -103,6 +103,9 @@ Label/annotation/env can by configured using either dict(key->value) or a file t
 deterministic) information when running with `--stamp` flag.  See the example in
 [/examples/labels/BUILD.bazel](https://github.com/bazel-contrib/rules_oci/blob/main/examples/labels/BUILD.bazel).
 
+Produces a target `[name].digest`, whose default output is a file containing the sha256 digest of the resulting image.
+This is similar to the same-named target created by rules_docker's `container_image` macro.
+
 
 **PARAMETERS**
 

oci/BUILD.bazel

@@ -50,6 +50,9 @@ bzl_library(
         "//oci/private:image_index",
         "//oci/private:push",
         "//oci/private:tarball",
+        "@aspect_bazel_lib//lib:copy_file",
+        "@aspect_bazel_lib//lib:directory_path",
+        "@aspect_bazel_lib//lib:jq",
         "@bazel_skylib//lib:types",
         "@bazel_skylib//rules:write_file",
     ],

oci/defs.bzl

@@ -10,6 +10,9 @@ load("//oci/private:tarball.bzl", _oci_tarball = "oci_tarball")
 load("//oci/private:image.bzl", _oci_image = "oci_image")
 load("//oci/private:image_index.bzl", _oci_image_index = "oci_image_index")
 load("//oci/private:push.bzl", _oci_push = "oci_push")
+load("@aspect_bazel_lib//lib:copy_file.bzl", "copy_file")
+load("@aspect_bazel_lib//lib:directory_path.bzl", "directory_path")
+load("@aspect_bazel_lib//lib:jq.bzl", "jq")
 load("@bazel_skylib//lib:types.bzl", "types")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
 
@@ -29,6 +32,9 @@ def oci_image(name, labels = None, annotations = None, env = None, **kwargs):
     deterministic) information when running with `--stamp` flag.  See the example in
     [/examples/labels/BUILD.bazel](https://github.com/bazel-contrib/rules_oci/blob/main/examples/labels/BUILD.bazel).
 
+    Produces a target `[name].digest`, whose default output is a file containing the sha256 digest of the resulting image.
+    This is similar to the same-named target created by rules_docker's `container_image` macro.
+
     Args:
         name: name of resulting oci_image_rule
         labels: Labels for the image config. See documentation above.
@@ -71,6 +77,29 @@ def oci_image(name, labels = None, annotations = None, env = None, **kwargs):
         **kwargs
     )
 
+    # `oci_image_rule` produces a directory as default output.
+    # Label for the [name]/index.json file
+    directory_path(
+        name = "_{}_index_json".format(name),
+        directory = name,
+        path = "index.json",
+    )
+
+    copy_file(
+        name = "_{}_index_json_cp".format(name),
+        src = "_{}_index_json".format(name),
+        out = "_{}_index.json".format(name),
+    )
+
+    # Matches the [name].digest target produced by rules_docker container_image
+    jq(
+        name = name + ".digest",
+        args = ["--raw-output"],
+        srcs = ["_{}_index.json".format(name)],
+        filter = """.manifests[0].digest | sub("^sha256:"; "")""",
+        out = name + ".json.sha256",  # path chosen to match rules_docker for easy migration
+    )
+
 def oci_push(name, remote_tags = None, **kwargs):
     """Macro wrapper around [oci_push_rule](#oci_push_rule).