So I provide my answer…
What I did to connect the guacamole docker containers to the default docker bridge:
root@ucs-member:/var/lib# docker network connect bridge guacamole_guacd_1
root@ucs-member:/var/lib# docker network connect bridge guacamole_guacamole_1
root@ucs-member:/var/lib# docker network disconnect guacamole_default guacamole_guacd_1
root@ucs-member:/var/lib# docker network disconnect guacamole_default guacamole_guacamole_1
root@ucs-member:/var/lib# univention-app restart guacamole
So far so good. That solved the issues described in this thread.
Next problem, and probably after some reading under https://docs.docker.com/network/network-tutorial-standalone/#use-user-defined-bridge-networks one reason why there is a user-defined bridge, is the following error, when I want to use a defined connection:
root@ucs-member:/var/lib# docker logs -ft --details guacamole_guacamole_1
...
2019-03-28T21:35:37.077055728Z 22:35:37.076 [http-nio-8080-exec-6] ERROR o.a.g.s.GuacamoleHTTPTunnelServlet - HTTP tunnel request failed: java.net.UnknownHostException: guacd: Name or service not known
...
I managed to get around this by editing /etc/hosts inside the guacamole container. But I’m not sure if this solution will even survive a restart.
Guacamole is working again, but I don’t think that this is a stable solution.
At least the guacamole_default bridge needs some repair from the maintainer I guess.
In univention bugzilla you can find a bug I think is related: Multicontainer apps use arbitrary network, database access issue, possible address conflicts
Best regards,
Bernd