Remove step to install Bundler in Dockerfile

I think we could have already done so when upgrading Ruby to version
2.6.x (which also included the Bundler gem), but since we didn't, now
that we've upgraded to Bundler 2.x it's probably a good moment.

Dockerfile

@@ -34,9 +34,6 @@ COPY Gemfile.lock Gemfile.lock
 
 COPY Gemfile_custom Gemfile_custom
 
-# Prevent bundler warnings; ensure that the bundler version executed is >= that which created Gemfile.lock
-RUN gem install bundler
-
 # Finish establishing our Ruby environment
 RUN bundle install --full-index