Fix streaming status to the docker client while pushing images

Pushing an Image causes the docker client to give an error message instead of
writing out streamed status.

This is caused by a Buffering message that is not in the correct json format:

[...]
{"status"
:"Pushing 6bba11a28f1ca247de9a47071355ce5923a45b8fea3182389f992f4
24b93edae"}Buffering to disk 244/? (n/a)..
{"status":"Pushing",[...]

The "Buffering to disk" message is originated in
srv.runtime.graph.TempLayerArchive

As the output is an incremental buffering display it is not usable for a
streaming result and should be either turned off or handed over to the std out
of the process.