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

/tmp is often a tmpfs file system and large temporary files could cause
docker commands to fail.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
What has this got to do with tmpdir? I presume `dir string` would be more what you mean?