Allow modifying the MediaType of a layer during mutate.Append. (#761)

pkg/v1/mutate/image.go

@@ -105,6 +105,10 @@ func (i *image) compute() error {
 			desc.URLs = add.URLs
 		}
 
+		if add.MediaType != "" {
+			desc.MediaType = add.MediaType
+		}
+
 		manifestLayers = append(manifestLayers, *desc)
 		digestMap[desc.Digest] = add.Layer
 	}

pkg/v1/mutate/mutate.go

@@ -40,6 +40,7 @@ type Addendum struct {
 	History     v1.History
 	URLs        []string
 	Annotations map[string]string
+	MediaType   types.MediaType
 }
 
 // AppendLayers applies layers to a base image.

pkg/v1/mutate/mutate_test.go

@@ -146,6 +146,7 @@ func TestAppendWithAddendum(t *testing.T) {
 		Annotations: map[string]string{
 			"foo": "bar",
 		},
+		MediaType: types.MediaType("foo"),
 	}
 
 	result, err := mutate.Append(source, addendum)
@@ -181,6 +182,9 @@ func TestAppendWithAddendum(t *testing.T) {
 	if diff := cmp.Diff(m.Layers[1].Annotations, addendum.Annotations); diff != "" {
 		t.Fatalf("the appended Annotations is not the same (-got, +want) %s", diff)
 	}
+	if diff := cmp.Diff(m.Layers[1].MediaType, addendum.MediaType); diff != "" {
+		t.Fatalf("the appended MediaType is not the same (-got, +want) %s", diff)
+	}
 }
 
 func TestAppendLayers(t *testing.T) {