Moved from separate project to ghcjs

Originally at https://github.com/mostalive/docker-ghcjs

  builds are at https://registry.hub.docker.com/u/atddio/docker-ghcjs/ .

This dockerfile (with ghcjs cloned from git instead of locally) built on
November 18. Changed the readme on the 21st and then the build broke,
cabal can't make a build plan.
Merge pull request #272 from atddio/master

Dockerfile to automatically build ghcjs