Add contrib/nuke-graph-directory.sh script to clean out old /var/lib/docker directories much more easily

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Merge pull request #7423 from tianon/nuclear-_-weapons--we-got-the-bomb

Add contrib/nuke-graph-directory.sh script to clean out old /var/lib/docker directories much more easily