I forgot to mention, the Phusion guys (who Mitchell mentions in the post and who create the excellent Passenger web server) have created some great assets for Vagrant and Docker:

https://github.com/phusion/baseimage-docker

https://github.com/phusion/passenger-docker (built on the baseimage above)

http://blog.phusion.nl/2013/11/08/docker-friendly-vagrant-bo...

And regarding role-based containers, Phusion's Hong Lai says:

"Wait, I thought Docker is about running a single process in a container?

Absolutely not true. Docker runs fine with multiple processes in a container. In fact, there is no technical reason why you should limit yourself to one process - it only makes things harder for you and breaks all kinds of essential system functionality, e.g. syslog.

Baseimage-docker encourages multiple processes through the use of runit."