oci: allow zstd for Docker images

Allow to use the zstd compression for docker images now that Docker
supports it: moby/moby#41759

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

image/oci.go

@@ -216,7 +216,7 @@ func (m *manifestOCI1) convertToManifestSchema2(_ context.Context, _ *types.Mani
 		case imgspecv1.MediaTypeImageLayerGzip:
 			layers[idx].MediaType = manifest.DockerV2Schema2LayerMediaType
 		case imgspecv1.MediaTypeImageLayerZstd:
-			return nil, fmt.Errorf("Error during manifest conversion: %q: zstd compression is not supported for docker images", layers[idx].MediaType)
+			layers[idx].MediaType = manifest.DockerV2Schema2LayerMediaTypeZstd
 		default:
 			return nil, fmt.Errorf("Unknown media type during manifest conversion: %q", layers[idx].MediaType)
 		}

image/oci_test.go

@@ -486,7 +486,7 @@ func TestConvertToManifestSchema2AllMediaTypes(t *testing.T) {
 	_, err := original.UpdatedImage(context.Background(), types.ManifestUpdateOptions{
 		ManifestMIMEType: manifest.DockerV2Schema2MediaType,
 	})
-	require.Error(t, err) // zstd compression is not supported for docker images
+	require.NoError(t, err)
 }
 
 func TestConvertToV2S2WithInvalidMIMEType(t *testing.T) {

manifest/manifest.go

@@ -24,6 +24,8 @@ const (
 	DockerV2Schema2ConfigMediaType = "application/vnd.docker.container.image.v1+json"
 	// DockerV2Schema2LayerMediaType is the MIME type used for schema 2 layers.
 	DockerV2Schema2LayerMediaType = "application/vnd.docker.image.rootfs.diff.tar.gzip"
+	// DockerV2Schema2LayerMediaTypeZstd is the MIME type used for schema 2 layers that use the zstd compression.
+	DockerV2Schema2LayerMediaTypeZstd = "application/vnd.docker.image.rootfs.diff.tar.zst"
 	// DockerV2SchemaLayerMediaTypeUncompressed is the mediaType used for uncompressed layers.
 	DockerV2SchemaLayerMediaTypeUncompressed = "application/vnd.docker.image.rootfs.diff.tar"
 	// DockerV2ListMediaType MIME type represents Docker manifest schema 2 list