bundler 2.0.1 issue about boarding HOT 9 OPEN

Many people are hitting issue which is only triggered when a new Bundler version is released. It happens because of a bug in Rubygems, and can be fixed by upgrading to 2.5.5+ or 2.6.3+.

Bundler 2.0.1
1) If you are deploying with Ruby 2.6.0 then you’ll get an error when your dyno or a release phase command trys to execute a bundle command:

remote: Bundler::LockfileError: You must use Bundler 2 or greater with this lockfile.
remote:   /app/vendor/ruby-2.6.0/lib/ruby/2.6.0/bundler/lockfile_parser.rb:108:in `warn_for_outdated_bundler_version'
remote:   /app/vendor/ruby-2.6.0/lib/ruby/2.6.0/bundler/lockfile_parser.rb:95:in `initialize'
remote:   /app/vendor/ruby-2.6.0/lib/ruby/2.6.0/bundler/definition.rb:83:in `new'

This is due to the built-in bundler integration that ships with Ruby 2.6.0. This bug is fixed in 2.6.1. If you hit the above error message with bundler 2.0.1, then please upgrade your Ruby version to use 2.6.1.

2) When using binstubs to run a command instead of bundle exec the wrong version of Bundler can get activated when using Ruby 2.6.x. This bug is reported to Ruby Core and will be fixed when Ruby 2.6.3 is released.

In the short term, the workaround is to ensure all commands in the Procfile and app.json are prefaced with bundle exec. For example:

web: bundle exec bin/rails server -p $PORT -e $RAILS_ENV
worker: bundle exec sidekiq -C config/sidekiq.yml
Technically this bug was fixed with 2.6.2 but there are other bugs with that version so we recommend 2.6.3+.

3) Ruby 2.5.x does not work with Bundler 2 unless the EXACT same version of bundler is specified in the Gemfile.lock.