sources/skopeo: fetch index manifest for dir

When the format is specified as "dir", copy the manifest specified in
the source digest and merge it into the final image directory.  The
effect of this is that the digest of the final image in the container
registry will match the manifest digest.  This enables users to specify
an image's manifest digest or a multi-image manifest list digest which
will be preserved in the container store on the final OS and allow them
to run the container using the same digest that was specified in the
input.

This may become a feature of Skopeo (or other tooling) in the future.
See containers/skopeo#1935

sources/org.osbuild.skopeo

@@ -93,9 +93,9 @@ class SkopeoSource(sources.SourceService):
             os.chmod(archive_dir, 0o755)
 
             source = f"docker://{imagename}@{digest}"
-
             archive_name = containers.archive_name(archive_format)
-            destination = f"{archive_format}:{archive_dir}/{archive_name}"
+            dest_path = os.path.join(archive_dir, archive_name)
+            destination = f"{archive_format}:{dest_path}"
 
             extra_args = []
 
@@ -119,6 +119,10 @@ class SkopeoSource(sources.SourceService):
                 raise RuntimeError(
                     f"Downloaded image {imagename}@{digest} has a id of {downloaded_id}, but expected {image_id}")
 
+            if archive_format == "dir":
+                # fetch the manifest and merge it into the archive
+                self.merge_manifest(source, dest_path)
+
             # Atomically move download archive into place on successful download
             with ctx.suppress_oserror(errno.ENOTEMPTY, errno.EEXIST):
                 os.makedirs(f"{self.cache}/{image_id}", exist_ok=True)
@@ -128,6 +132,23 @@ class SkopeoSource(sources.SourceService):
         archive_name = containers.archive_name(desc["image"].get("format", "docker-archive"))
         return os.path.exists(f"{self.cache}/{checksum}/{archive_name}")
 
+    def merge_manifest(self, source, destination):
+        with tempfile.TemporaryDirectory(prefix="tmp-download-", dir=self.cache) as indexdir:
+            # download the manifest(s) to a temporary directory
+            subprocess.run(["skopeo", "copy", "--multi-arch=index-only", source, f"dir:{indexdir}"],
+                           encoding="utf-8", check=True)
+
+            # calculate the checksum of the manifest of the container image in the destination
+            manifest_path = os.path.join(destination, "manifest.json")
+            with open(manifest_path, encoding="utf-8") as manifest:
+                manifest_checksum = hashlib.sha256(manifest.read().encode()).hexdigest()
+
+            # rename the manifest to its checksum
+            os.rename(manifest_path, os.path.join(destination, manifest_checksum + ".manifest.json"))
+
+            # move the index manifest into the destination
+            os.rename(os.path.join(indexdir, "manifest.json"), manifest_path)
+
 
 def main():
     service = SkopeoSource.from_args(sys.argv[1:])