Example: Run bundle package only on Gemfile change

If you can cache your vendor/ directory between runs, this allows you
to skip the package step if neither Gemfile nor Gemfile.lock have
changed.

This also allows using the Docker cache for building the image when
using Gems fetched via Git. `bundle package` for some reason updates
the Git repositories' main folder's change time, so Docker does a full
rebuild. When not doing `bundle package`, the Git repositories don't
change and the Docker cache works.