Moved the data volumes to a data-only container. 

This will persist content and users in the event of the main ghost container being killed and restarted.

Currently the `VOLUME ['/data/ghost/content/images', '/data/ghost/content/data']` instruction will mount the  container's data somewhere in `/vol/lib/docker/volumes`. However, if the main container is killed and restarted there's no easy way for the new container to access the data. The most dockerish way to solve this problem is to create a "data-only" container. Its not completely intuitive but this [blogpost](http://www.offermann.us/2013/12/tiny-docker-pieces-loosely-joined.html) does a great job of explaining the idea in detail.