Gitbase bind mounts also creates volumes, prune does not clean them up

[carlosms]

Reported here https://github.com/src-d/engine/pull/351#pullrequestreview-218737410
A docker inspect shows that gitbase contains 2 bind mounts + 2 anonymous volumes.
When the container is stopped the volumes are not cleaned up, and srcd prune also leaves them.

My guess is that we should not be creating cfg.Volumes for type bind, here
https://github.com/src-d/engine/blob/master/docker/docker.go#L304