docker save: Do not save to a terminal.

Pretty straightforward, something I noticed while triaging an issue.

The "cowardly refusing" is a hat-tip to gzip which says something similar when you try to do this.
Yeah, that’s why I did it this way.

On Sep 30, 2014, at 5:51 PM, Jessie Frazelle <notifications@github.com> wrote:

> In integration-cli/docker_cli_save_load_test.go:
> 
> > @@ -60,6 +63,33 @@ func TestSaveAndLoadRepoStdout(t *testing.T) {
> >  
> >  	logDone("save - save a repo using stdout")
> >  	logDone("load - load a repo using stdout")
> > +
> oh nm then we would have to make the repo again
> 
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