Update README.md install instructions, add Docker support.

[toolness]

This updates the README.md with the following info for normal development flows:

• Ruby 2.4 doesn't work with this project; apparently one of our dependencies uses a version of the json gem that isn't compatible with 2.4 (see Please release a gem new version for Ruby 2.4 compatibility ruby/json#286 for more details). We could probably fix our Gemfile to make it work with 2.4 somehow, but for now I figured I'd just write a disclaimer to use Ruby 2.3.
• Node is actually required for jekyll_pages_api_search to work. Fortunately, the site somewhat gracefully degrades to not support search if Node can't be found, rather than crashing.

This PR also adds a Docker setup so that folks who don't want to deal with installing the right version of Ruby and Node can just docker-compose up and go.

[tadhg-ohiggins]

Looks good, thanks for adding this! (Also, sorry for the review delay.)