handle symlinks for Docker's root dir & TMPDIR

This removes the incomplete symlink handling from engine.go and it adds
it one place in docker.go.

It also enables handling symlinks for TMPDIR.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #4382 from unclejack/fix_symlink_handling

handle symlinks for Docker's root dir & TMPDIR