Add a Docker Compose file that launches a minimal Danbooru instance in a
Docker container with a single command. This is suitable as a quick demo
or for personal use, not for public-facing sites.
To use it, just run `bin/danbooru`. This is a wrapper script that
installs Docker Compose then uses it to start Danbooru.
This will generate a lot of debug output and take several minutes while
it builds the Docker containers. Be patient. When it's done, you should
have an empty booru accessible at http://localhost.
Comparison:
* Codecov has a simpler integration and a better UI.
* Codeclimate tracks both linter warnings (Rubocop, ESLint) and code
coverage, but its UI for code coverage is worse than Codecov's.
* Codeclimate doesn't support Simplecov 0.18 because Codeclimate doesn't
support 0.18's new coverage format yet.
