Docker setup improvements

[rodolfobandeira]

Proposal:

• Speed up bundle install by adding jobs=2 config
• Add a bundle cache volume so we don't need to install fresh gems on every docker iteration

[exegeteio]

Regarding bundle cache: Is the intent to speed up docker build by having the gem cache?

As best I can tell, Docker already includes caching in the build process so that the Gems should not be installed unless Gemfile or Gemfile.lock are updated. Not sure the problem that part is trying to fix.