I have no experience with Plesk Docker but from what I see is that the manager cannot find keycloak service. Are you starting the whole stack with the provided docker-compose.yml? Some default environment variables values are set there. Also note that http://keycloak:8080/auth is on internal docker network and has to be resolved by docker itself. This can be a difference between Plesk Docker and standard docker. There is also a way to start the stack without keycloak.