Docker: Set bind mount propagation.

This changes the mount propagation for the bind mount of the Gluster volume directory from the default private propagation, where later mounts or unmounts of the source directory are not propagated to the target, to "rslave," where they are. This is necessary so that the container's accessing the mount triggers the host's automounter and so that a mount performed by the automounter after the container starts is accessible from the container. This also makes it so that the container's mount is automatically unmounted when it's idle, often saving a step when performing a fail-over of the Gluster cluster.