feat: support Image Index with a subject (#548)

Resolves: #531
Signed-off-by: Lixia (Sylvia) Lei <lixlei@microsoft.com>

content/graph.go

@@ -75,18 +75,33 @@ func Successors(ctx context.Context, fetcher Fetcher, node ocispec.Descriptor) (
 		}
 		nodes = append(nodes, manifest.Config)
 		return append(nodes, manifest.Layers...), nil
-	case docker.MediaTypeManifestList, ocispec.MediaTypeImageIndex:
+	case docker.MediaTypeManifestList:
 		content, err := FetchAll(ctx, fetcher, node)
 		if err != nil {
 			return nil, err
 		}
 
-		// docker manifest list and oci index are equivalent for successors.
+		// OCI manifest index schema can be used to marshal docker manifest list
 		var index ocispec.Index
 		if err := json.Unmarshal(content, &index); err != nil {
 			return nil, err
 		}
 		return index.Manifests, nil
+	case ocispec.MediaTypeImageIndex:
+		content, err := FetchAll(ctx, fetcher, node)
+		if err != nil {
+			return nil, err
+		}
+
+		var index ocispec.Index
+		if err := json.Unmarshal(content, &index); err != nil {
+			return nil, err
+		}
+		var nodes []ocispec.Descriptor
+		if index.Subject != nil {
+			nodes = append(nodes, *index.Subject)
+		}
+		return append(nodes, index.Manifests...), nil
 	case spec.MediaTypeArtifactManifest:
 		content, err := FetchAll(ctx, fetcher, node)
 		if err != nil {