glowing-bear-ssl-proxy should not rely on docker IP addresses

[tgymnich]

glowing-bear-ssl-proxy should not rely on internal docker IP addresses since they might be different on some systems. Using dns names and docker networks is more flexible and reliable. I suggest using the docker networks defined in docker-compose.yml and keycloak.yml. This can also reduce the number of ports exposed to the outside and thus improve security.

External networks can be used like this:

networks:
  default:
    external:
      name: my-pre-existing-network

[gijskant]

Sounds like a good improvement. Indeed, using the internal Docker IP address feels like a hack. We were not aware of the possibility to refer to an external network.
Could you create a PR for this?