I think it's kind of okay to commit credentials and configuration used for the local dev environment (and ONLY the local one).

No. Never.

E.g. when you require some infrastructure like a database inside a container for your app. Not every dev wants to manually set a few dozen configuration entries when they quickly want to checkout and run the app

In this situation, it would be better to write a simple script that can generate fresh and unique values for the dev.

Laziness is not an excuse.