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.
@erikh 
Let's try to use the `bytes` package wherever possible. Also, `buf` has 1024 length even if Read did not read 1024 bytes. I suggest the following:
```Go
n, err := pty.Read(buf)
if err != nil { ... }
if !bytes.Contains(buf[:n], []byte("Cowardly refusing")) { ... }
```