Onbuild images really only work on the Debian based images since they are using buildpack-deps which gives us many libraries so that most anything installed from requirements.txt just works. The alpine images are meant to be super tiny so a user will need to install any necessary libraries or build tools before doing a pip install. We really only recommend onbuild as a starting point where most users should probably use one of the other variations and be able to add in their own custom steps.