imagetools(create): set correct media type when combining manifests

When using imagetools create and combining multiple sources
we should check the media type of each manifest and set
the right media type for the manifest list.

If there is a mismatch we set OCI index as best effort.

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

util/imagetools/create.go

@@ -122,24 +122,29 @@ func (r *Resolver) Combine(ctx context.Context, srcs []*Source) ([]byte, ocispec
 		}
 	}
 
-	mt := images.MediaTypeDockerSchema2ManifestList //ocispec.MediaTypeImageIndex
-	idx := struct {
-		// MediaType is reserved in the OCI spec but
-		// excluded from go types.
-		MediaType string `json:"mediaType,omitempty"`
-
-		ocispec.Index
-	}{
-		MediaType: mt,
-		Index: ocispec.Index{
-			Versioned: specs.Versioned{
-				SchemaVersion: 2,
-			},
-			Manifests: newDescs,
-		},
+	dockerMfsts := 0
+	for _, desc := range newDescs {
+		if strings.HasPrefix(desc.MediaType, "application/vnd.docker.") {
+			dockerMfsts++
+		}
 	}
 
-	idxBytes, err := json.MarshalIndent(idx, "", "  ")
+	var mt string
+	if dockerMfsts == len(newDescs) {
+		// all manifests are Docker types, use Docker manifest list
+		mt = images.MediaTypeDockerSchema2ManifestList
+	} else {
+		// otherwise, use OCI index
+		mt = ocispec.MediaTypeImageIndex
+	}
+
+	idxBytes, err := json.MarshalIndent(ocispec.Index{
+		MediaType: mt,
+		Versioned: specs.Versioned{
+			SchemaVersion: 2,
+		},
+		Manifests: newDescs,
+	}, "", "  ")
 	if err != nil {
 		return nil, ocispec.Descriptor{}, errors.Wrap(err, "failed to marshal index")
 	}