remotes/docker: Add MountedFrom and Exists push status

This makes it possible to check whether content didn't actually need to
be pushed to the remote registry and was cross-repo mounted or already
existed.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

remotes/docker/status.go

@@ -36,6 +36,17 @@ type Status struct {
 
 	// UploadUUID is used by the Docker registry to reference blob uploads
 	UploadUUID string
+
+	// PushStatus contains status related to push.
+	PushStatus
+}
+
+type PushStatus struct {
+	// MountedFrom is the source content was cross-repo mounted from (empty if no cross-repo mount was performed).
+	MountedFrom string
+
+	// Exists indicates whether content already exists in the repository and wasn't uploaded.
+	Exists bool
 }
 
 // StatusTracker to track status of operations