After some stabbing in the dark, found this (undocumented) syntax to work:
[build]
builtin = "python"
[build.settings]
pythonbase = "3.9-slim-buster"
After some stabbing in the dark, found this (undocumented) syntax to work:
[build]
builtin = "python"
[build.settings]
pythonbase = "3.9-slim-buster"