My team noticed a bit of a gap when it comes to documentation and setting up Bitbucket Pipelines when you have some modern webpack dependencies with yarn. So, we wrote some instructions explaining how we navigated our way through this process.

https://blog.planetargon.com/entries/how-to-set-up-bitbucket-pipelines-on-a-rails-52-environment-with-react_on_rails-and-webpack