better fig and docker setup

Attempting to provide a better fig / docker experience, I've converted the fig setup to build a local image (based of jruby image) and insert our code into it:

1.  Added a script to try and avoid the cost of reinstalling gems each time by using `fig up --no-recreate` on the docker containers. 
2.  Added a script to run bash commands a fig container via fig up with an overridden entrypoint (which needs to stay due to our prod env).
3.  Added documentation to the readme to help people get started.
Or maybe put bundle install/DB migration in a separate script?