I think I figured it out. In case it helps somebody:
It’s necessary to rename config.php.dist to config.php, and debug.php.dist to debug.php, both in the top level of the source directory. Renaming debug.php ensures that webapp is run in “source mode” where the source code is loaded directly from the source directory, with no build steps (and no ant/java packages) necessary.
I made a simple Docker image based on Debian which I can make available if there is any interest.