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.
Probably - the default fig_run_cmd gets a rails console but it probably should dump out without a string command and not overload the definition. 

I will add the fig dir to scripts and look at splitting the run command out and adding the migrate command.