I was missing an important part of setting up a local joplin-server.
Probably it is a nice add on in the doc.
For running on an Ubuntu 24.04.4 LTS (Noble Numbat) I missed the follwing settings to get the server up and running as a docker container.
I do not know if it applies inly to ubuntu but I could imagen rather not.
I did not see it in the joplin-server's README.md, but it was necessary.
Before the change all file rights were given to my user which is the administrative user of the os.
# Change ownership of the data folder to the ID Joplin uses
sudo chown -R 1001:1001 ~/joplin-server/data
# Ensure the folder has write permissions
sudo chmod -R 755 ~/joplin-server/data