Merge pull request #990 from cloudfoundry/dockerfile-ruby-fix

Dockerfile ruby fix

Author: pivotal-david-osullivan

ci/Dockerfile

@@ -26,7 +26,7 @@ RUN eval "$(rbenv init -)" \
 
 RUN eval "$(rbenv init -)" \
  && git clone https://github.com/sstephenson/rbenv-default-gems.git $(rbenv root)/plugins/rbenv-default-gems \
- && echo 'bundler' >> $(rbenv root)/default-gems
+ && echo 'bundler 2.3.26' >> $(rbenv root)/default-gems
 
 RUN eval "$(rbenv init -)" \
  && rbenv install 2.5.9