Docker should use /var/lib/container/tmp for large temporary files.

/tmp is often a tmpfs file system and large temporary files could cause
docker commands to fail.  Also using /tmp potentially allows users on the
system to get access to content, or even attack the content.  Moving the tmpdir to
/var/lib/container/tmp will protect the data.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

Conflicts:
	docker/docker.go
Merge pull request #6456 from rhatdan/tmpdir

Docker should use /var/lib/docker/tmp for large temporary files.