add github.com/docker/docker dependency

vendor/github.com/docker/distribution/manifest/versioned.go

@@ -0,0 +1,12 @@
+package manifest
+
+// Versioned provides a struct with the manifest schemaVersion and mediaType.
+// Incoming content with unknown schema version can be decoded against this
+// struct to check the version.
+type Versioned struct {
+	// SchemaVersion is the image manifest schema that this image follows
+	SchemaVersion int `json:"schemaVersion"`
+
+	// MediaType is the media type of this schema.
+	MediaType string `json:"mediaType,omitempty"`
+}