This is not a support request, just a comment for others and myself:
I installed ODK Central 0.9 on a new Ubuntu 20.04 installation, and selected the snap-version of Docker because I thought it would be less work.
This gave an Error: /data/transfer is on a read-only file system. The solution was in an thread from 2018: https://stackoverflow.com/questions/52526219/docker-mkdir-read-only-file-system
Trying to remove the Docker snap failed because I could not unhang it (yes, I searched all relevant google. stuff, the solutions did not work, rebooting a dozen of time neither). So I had to bite the bullet and reinstall Ubuntu.
Error was gone after I used the manual installation of Docker and docker-compose.