Add a Dockerfile

[rkdarst]

• I wouldn't say I like docker, but I didn't want to install npm on my
  own computer to build this, so created a quick Dockerfile. It seems
  to work for me, so I may as well make it useful to others.
• This is capable of running as-is, but I also used it to extract the
  built files out and host them elsewhere.
• I now realize there was a simpler way to do this (get built files
  from the gh-pages branch), so I probably won't use this anymore.
  Still, maybe useful to someone? I have no problems if this is
  rejected as extra complexity.
• Review
  • I wouldn't call myself a docker expert, so a review of that aspect
    would be welcome.