Health check Mendix 6.9.1 in Docker environment

We want to use the built in Mendix health check, but can't get it to work.

In short the following configuration in the DCOS deployment JSON file:

"docker": {
            "image": "Mx_docker_image",
            "network": "BRIDGE",
            "portMappings": [
              {
                    "containerPort": 9000,
                    "protocol": "tcp",
                    "labels": {
                        "VIP_1": "/xxxx:9000"
                    }
                }
            ],
        }

  "env": {
        "M2EE_admin_port": "9000",
        "M2EE_admin_pass": "P@ssword!",
        "M2EE_admin_listen_addresses": "127.0.0.1",
}

So the other variables defined here do work, but we can't find the port 9000 to be open.

Tested by going into the linux machine console and executing:

ss -tulpen

(command to show all open ports).

Are we doing something wrong? Is there a guide to use the Mendix built in healthchecks when running it on a Docker container?