Ruby's docker images (e.g. ruby:2.6.3) use bundler v1 (e.g.
1.17.3), see: https://github.com/docker-library/ruby/issues/268#issuecomment-469491078
When bundling with bundler v2 locally it can lead to the the following exception:
Could not find 'bundler' (2.0.2) required by your /app/Gemfile.lock. (Gem::GemNotFoundException)
Solve this by using the correct bundle version locally:
gem install bundler -v 1.17.3