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.

Fixes #4396

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)
I also think it makes more sense to store the RootDir variable not in utils, but in the `var` section of this file, just like `dockerConfDir`, unexported so `rootDir`. Then you could have utils.TempDir take the rootDir as a parameter. It just feels wrong to have the RootDir variable in the utils package. Let me know if you disagree.