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.