Provide instructions for 'docker-in-docker' for docker based planemo machine

Instructions should be added to the deployment of the docker image of the planemo box to instruct the user how to mount the parent OS docker server so the box can deploy sub-containers.

I've used:
--privileged -v /var/run/docker.sock:/var/run/docker.sock

Which mounts in socket to the parent OS's docker server (still need to verify that privileged is necessary)