Start mesos-slave after docker when present
Start mesos-slave after docker when present

If mesos-slave is configured with --containerizers=docker, it will fail
to start at boot time if it cannot communicate with the Docker daemon.
Add a 'Should-Start' dependency on the docker service so that startup is
appropriately delayed on systems using the SYSV init mesos-slave script.

jaytaylor/mesos-deb-packaging,justenwalker/mesos-deb-packaging,deric/mesos-deb-packaging,neilconway/mesos-deb-packaging