[keycloak-user] Docker and Port 9999

Hi, I am still trying to get the Domain Clustered Mode in Docker working. My Master container runs and is callable from the host, but I have problems to connect the slave. The port 9999 is needed from the slave, right? I exposed it in the masters dockerfile, but if I do nmap from the slave it is still shown as closed. What can I do to open it? Best regards Marc