Use docker

Docker support for development.

* added Dockerfile to support the creation of Docker containers
* updated README with information on how to setup a Docker container
I know that the Docker-based tests in http://hg.mozilla.org/hgcustom/version-control-tools/ require me to copy & paste some environment variable definitions from the boot2docker output. That's probably the better option here; I set those variables in my local environment and don't have to edit any files.