Add -V flag to docker run to create non-existent bind-mounted host dirs (fixes #1279)

By default, Docker will fail if the host-side source directory of a bind mount does not exist, nor will it create it. This is a good thing because it makes sure spurious directories are not created all over the place on the host when the command is mis-used.

There are cases where having Docker automatically create the host-side directory for a bind mount can be desirable though. This change adds a new -V flag that will allow Docker to create the missing directories when needed.