Using data volume containers for caching gems

Keeping the dependencies outside of the app container allows us to update the
Gemfile without the need for re-building the container again; this speeds up
bundle install and bundle update calls.

By keeping them in a data volume container we are able to reuse them across
multiple projects, which allows us to save disk space.

This approach aims to solve the bundle install issue of the previous version.
Because it relies on data volume containers that have to be created outside of
Fig (so the fig rm doesn't remove them), a script file is provided to aid the
process of bootstrapping a project (project.sh). The README.md was updated to
provide with the needed instructions and the fig.yml file adapted accordingly.