As we mentioned in the readme, the builder project is based on gcp/buildpacks with some modifications to facilitate support for Chinese users and support for OpenFunction's functions-framework. gcp/buildpacks primarily serves GCP, so we may need to do more work on improving builder customization.
Of course, we are also considering rewriting a Builder project that will still follow the buildpacks lifecycle criteria (which drove us to find out more about the builder's improvement needs).