aparently the new Opencast docker images are based on Ubuntu 22. If you use Ubuntu 20 as your host system, and the default docker package sources provided by Cannonical (i.e. not those provided by Docker), you'll probably still have Docker version 19.x
During startup while invoking java the first time (try_to_connect_to_db), you'll probably see
A simple workaround was removing docker provided by Ubuntu, switching to the docker sources.