Adding basic support for vagrant docker provider.

This gives the minimum support for being able to create the container
using vagrant: `vagrant up --provider=docker`
It's especially useful for OS X, as you can't run containers natively,
so vagrant will create a host VM running docker to run those containers.

Additionally, fix some issues where compass fails to build on the
container, due to the build tools (build-essentials and ruby-dev)
not being available.

Hope it's useful :)

There's a blog post on the vagrant site that might also be useful: https://www.vagrantup.com/blog/feature-preview-vagrant-1-6-docker-dev-environments.html