Add docker support using jekyll's official container

[toolness]

This is a redo of #269 which follows @MattSurabian's advice and uses the official Jekyll Docker image instead of rolling our own thingy.

Notes

• We disable the jekyll-minifier plugin during development because it adds an enormous amount of overhead: building the site goes from 0.5-1.5 seconds on my machine to over 11 seconds with this plugin enabled. Obviously it's not needed for development, so I disabled it via a _config_dev.yml file that the Docker setup includes via Jekyll's --config argument.

To do

• [x] Add documentation.

[iamjessklein]

This looks great to me @toolness - I really like your word choice. Saying things like "if you are comfortable with" makes this more approachable to junior coders. 👍