Optimize the fig dev env/build

Closes #344 - not ready to merge just yet, can someone check the README instructions make sense and work before merging?

So i tried using a persistent data volume beneath panoptes but `fig up` would recreate the containers on each launch, whic was no better than the existing setup with `fig up --no-recreate`. The solution I've settled on is to switch in a development dockerfile (with dev/test gems) before `fig build` to ensure all gems are installed. Then just manually migrate and seed the db before using `fig up`. This alleviates the need to install gems / migrate the DB on each run and we can recreate containers each time with minimal work to get started.