remotes/docker: Only return "already exists" on push when the upload was successful

The `(dockerPusher).Push` method uses a `StatusTracker` to check if an
upload already happened, before repeating the upload. However, there is
no provision for failure handling. If a PUT request returns an error,
the `StatusTracker` will still see the upload as if it happened
successfully. Add a status boolean so that only successful uploads
short-circuit `Push`.

Signed-off-by: Aaron Lehmann <alehmann@netflix.com>

remotes/docker/status.go

@@ -28,6 +28,8 @@ import (
 type Status struct {
 	content.Status
 
+	Committed bool
+
 	// UploadUUID is used by the Docker registry to reference blob uploads
 	UploadUUID string
 }