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.
Yes - first detects it hasn't been run before and runs the bundle/jbundle install and migrates the DB.
Second it tries to boot. I could have it just boot after migrating the DB if we want.