I think there is rarely a reason to choose this over just a standard Dockerfile, which is usually not that complicated and probably wont change much over time o

I think there is rarely a reason to choose this over just a standard Dockerfile, which is usually not that complicated and probably wont change much over time once it has been implemented. If generating Dockerfiles needs to be scaled up somehow, I can understand why this would be useful. Sounds exactly like a problem someone like Heroku would have.