Fix for issue #3786 `docker rm` unmounts in the wrong order

This is a fix for the case that one mount is inside another mount and docker
can't then delete the resulting container.

Docker-DCO-1.1-Signed-off-by: Peter Waller <p@pwaller.net> (github: pwaller)

Fixes #3786

/cc @drj11

I've tested this, and it fixes the issue for the case reported in the original
bug.