My legendary persistence has paid off!
From looking at other Dockerfiles in the repo, I discovered that the JS files needed were not being copied to the image in the original Dockerfile I used to base this on. No wonder it was looking everywhere and not finding anything!
COPY ./build/*.js /nakama/data/modules/build/
Now my runtime code is printing “Hello World!” in the server log output.
The issue was the same for the config file.
COPY ./nakama-config.yml /nakama/data/
And now I see the config file was loaded successfully, too.