* Automate the build process, instead of writing a complicated manual
* Update sources used by the docker image away from google code
* Exclude MathJax-dev checkout from docker image, to allow image reuse
* Try to keep the individual image layers small by combining RUN commands
  and removing temporary files used only during each individual step

Also use hash of Dockerfile to compute image tag, in order to ensure
creation of a new image if the Dockerfile changes.