Fix the quickstart command in the README not working for Ubuntu 18.04.
This was because the Docker Compose file was set to version 3.7, but
Ubuntu 18.04 ships an older version of Docker Compose that only supports
version 3.4.
* Optimize Dockerfile to minimize size of the Docker image.
* Specify exact versions of important dependencies (Ruby, Node, Vips) to
ensure our dependencies are up to date and locked to known versions.
* Install Vips from source because the version that ships with Ubuntu is too old.
* Install FFmpeg from source because otherwise using the Ubuntu package
pulls in tons of video libraries we don't need, bloating the image.
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.
