It would be nice to add custom.js into index.html from customassets folder if the file exists in.
Based on the experience with Docker I would suggest not solving this scenario in this way. If I understand well, the request was to introduce a possibility to override custom.jsby providing one through the model repository.
My opinion is that any custom assets should be bundled using a custom docker image (even images and other files). We should create a topic that describes these scenarios and how should they be handled.