Support running CLI with Docker

[Andrew Louis]

Provide an easy way to run the tool for those who don't want to use rubygems or homebrew.

Todos:

• firm up dependencies in the gemspec
• automate the versioning in Dockerfile
• handle config/secrets/auth that currently assume access to xdg file structure

Annoying things:

• will need to run with docker run -it flags for proper tty sizing and stdin
• almost all jobs will require mounting the homedir (-v /Users/alouis:/Users/alouis) and a lot of plugins guess data locations based on username and homedir paths
• plugins won't stay installed between runs (maybe install all well-known plugins in dockerfile?)