Add a Dockerfile for huxley tests

Summary:
Add the dockerfile that I've been using to make huxley screenshots so
others can test their changes by making huxley screenshots. Include a readme
about how to use it correctly.

Test Plan: - Use the dockerfile by running the given commands, see it works

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12773