Install convoy volume driver

I build an quick and dirty, but working docker image to use with RancherOS system-docker.
http://forums.rancher.com/t/add-binary-to-rancheros-as-os-service/501/3

I have some improvements in mind, but need a way to start the convoy container before (user-)docker. I know how to start it after another container (just with a container link), but not how to start it before…