Fix for issue #3786

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)
Merge pull request #3787 from pwaller/issue-3786

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