How to build Rails app migrated with Turboku?

Hello! I just migrated a Rails app (refrigerator-poetry-forum) from Heroku to using Heroku · Launch on and it works great – I’m really impressed!

However, I’m confused about how to deploy changes to my app to directly – I saved a fly.toml using flyctl config save, but it doesn’t include a [build] section saying which builder to use to be compatible with Heroku.

I think I’m using the heroku16 buildpack, and that I need to add a [build] section to my fly.toml that says that, but I’m not sure exactly how to set up my fly.toml.