(The default is empty; if it's set then docker forwards the port.)
The nginx configuration will also need to be updated in order to deal with the additional JVB servers. This is achieved via its own configuration variable
`matrix_nginx_proxy_proxy_jitsi_additional_jvbs`, which contains a dictionary of server ids to ip addresses.
For example,
``` yaml
matrix_nginx_proxy_proxy_jitsi_additional_jvbs:
jvb-2: 192.168.0.2
jvb-3: 192.168.0.3
```
Applied together this will allow you to provision extra JVB instances which will register themselves with the prosody service and be available for jicofo