Isn't bundler _bundled_ with ruby?

https://blog.jetbrains.com/ruby/2017/10/10-new-features-in-ruby-2-5/

ruby/2.5-rc/alpine3.6/Dockerfile

Line 97 in 6959194

&& gem install bundler --version "$BUNDLER_VERSION" --force

[tianon]

Yep, see the discussion on #165 (comment).