work around bundler warnings
This fix for #7 (closed) is based on suggestions from
- https://bundler.io/v1.16/bundle_config.html
- https://blog.bigbinary.com/2016/02/09/configuring-bundler-using-bundle-config.html
- https://medium.com/@ddomachine/docker-and-ruby-on-rails-448ab8255f97
and is related to the known issues
as well as the other ones from previous conversation.