It was quite a struggle to get this to work. I've left traces of all the various things I either expected or found others documenting (all the devices in the docker compose doc, for example), but I left it in a state that was working at the time of this writing. I'm leaving it in a state where it was working for me on a Raspberry Pi 4.
To use:
docker compose up -ddocker exec libcam bashlibcamera-hello
have fun :)