Configure a build command for a Python project without using the web UI

I’d like to configure a build command for my Python application using configuration in my GitHub repository.

(I don’t want to use the UI for this because I want to create a template repository that other people can easily fork and alter for their own projects)

Is there a way of doing this directly using a file in the repo? I tried adding a release: block to my Procfile but that didn’t seem to work.

I know you can copy the App Spec into a .do/ directory in the repository but that’s no good for creating a template, because it bakes in details that are unique to your app (the GitHub repo and the app name).

I’d like an easy mechanism in my GitHub repository to say “run this build script” which will keep working if other people create copies of my repos and adapt them for their own projects.