[wip] Use a Docker container to build the site

I'm creating a Docker image that contains all of the build tools and configuration necessary to compile the static site. Hopefully, this can ease some of the pain we've seen lately related to unreliable Vagrant builds.

My goal is for an end user to be able to run `script/preview` and reliably see their local changes in a browser.

This is the first, tiniest tangible part of #974. #976 will obsolete this for most contributors, but this work may help set that up. Also, this will create a certain amount of redundancy in our deployment framework. My intention is to follow this up in a few sprints with modifications to the infrastructure to also use this container in our Jenkins build. I'm also using this to establish some useful conventions for Dockerfile organization and script support.

### Remaining Work

- [ ] Jekyll locale issues :rage:
- [ ] Automatically open the URL on Macs.
- [ ] Investigate live preview. It won't work for the Sphinx parts, but I may be able to get this working for the Jekyll parts.
- [ ] Try to cut down the Docker image from 973 MB.
- [ ] Change the README instructions.