My original post was unclear, I don’t think this is related to Koyeb at all.
I actually managed to track it down. It appears libyaml-dev was removed from the Ruby slim and alpine images two days ago, when this started happening. There’s a lot more information in that GH issue. I kind of got lucky as I was looking for the Rails Dockerfile template and noticed libyaml-dev is now included!
In answer to how can I be informed, it appears that’s a lot of the discussion on the GH issue page. In answer to how could this happen w/o a change on my end, I need to understand if there’s a versioning and locking system w/ base Docker images similar to semver as letting that float to latest is how this was introduced.