Docker run with non-root user fails

The documentation state that you can use a specific user rather than the default one:

Note : The container runs as the user 1000 and no group by default. You can use Docker’s --user flag to change that.

However, using Docker’s --user flag fails, as the container is no longer started as root and the container’s initialization step fails to create the user (which is still using id 1000:1000):

docker run   --name vikunja --restart unless-stopped --user 2713 -p 2713:2713 -v /home/vikunja/config.yml:/app/vikunja/config.yml:ro -v /home/vikunja/data/files:/app/vikunja/files -v /home/vikunja/data/logs:/app/vikunja/logs vikunja/vikunja
info: creating the new user vikunja with 1000:1000
groupmod: /etc/group.7: Permission denied

Is the documentation correct and is it indeed possible to run the container as non-root user ?

Does that work with an unstable build?