Create Docker container to host the xblock-sdk

This implements an initial Docker container to host an instance of the
OpenEdX XBlock SDK which is to be used during XBlock development.
Like Ubuntu, Debian (wheezy) doesn't have a default package for nodejs.
Unlike Ubuntu, the fix is a bit more involved [1]. Instead of `curl`ing
into `bash` (facepalm), we commit this to the repository.

Alternatively, we _could_ install from source..

[1] https://github.com/joyent/node/wiki/Installing-Node.js-via-package-manager#debian-and-ubuntu-based-linux-distributions