After regular docker installation you need to login into EGroupware Setup under the following URL:
Use the lower / header-admin login using user “admin” and the password from /var/lib/egroupware/egroupware-docker-install.log. Create the addtional Domain(s) and save the header.inc.php.
Log out and in again using the upper Login for one of the new domains, create the database, install it and create a first admin user for the instance.
The Container Update will keep all instances/domain up to date.