Automatically relabel volume mounts if user passes :Z with --volume

When you volume mount into a labeled container, we want to allow the
administrator/user the ability to tell docker to fix the labels on the mount.
Using the same basic syntax as the :ro and :rw options on the --volume command

:Z will set the labels so that only the processes within the container can
read/write content.  Since the labels change on creation, this can not be done
by the administrator prior to running the container

:z will change the labels so all containers could read/write content.

drwxr-xr-x. root root system_u:object_r:svirt_sandbox_file_t:s0 /var/run/dan

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)