We don't need the whole git history, so doing a shallow clone is
favourable. Cloning a specific release (the latest at the time of
writing this) allows us to have reproducible results since we're not
cloning whatever is on master at the time of building the Docker image.
The Ruby version was updated, but the Dockerfile wasn’t. This meant that
the Docker environment was broken. This change should prevent similar
breakage in the future by making .ruby-version the source of truth about
the correct version.
