Add information to make jitsi work with a client in LAN

docs/configuring-playbook-jitsi.md

@@ -49,6 +49,21 @@ matrix_jitsi_enable_auth: true
 matrix_jitsi_enable_guests: true
 ```
 
+## (Optional) Making your Jitsi server work on a LAN
+
+By default the Jitsi Meet instance does not work with a client in LAN (Local Area Network), even if others are connected from WAN. There are no video and audio. In the case of WAN to WAN everything is ok.
+
+The reason is the Jitsi VideoBridge git to LAN client the IP address of the docker image instead of the host. The [documentation](https://github.com/jitsi/docker-jitsi-meet#running-behind-nat-or-on-a-lan-environment) of Jitsi in docker suggest to add DOCKER_HOST_ADDRESS in enviornment variable to make it work.
+
+Here is how to do it in the playbook.
+
+Add these two lines to your `inventory/host_vars/matrix.DOMAIN/vars.yml` configuration:
+
+```yaml
+matrix_jitsi_jvb_container_extra_arguments:
+  - '--env "DOCKER_HOST_ADDRESS=<Local IP adress of the host>"'
+```
+
 ## Apply changes
 
 Then re-run the playbook: `ansible-playbook -i inventory/hosts setup.yml --tags=setup-all,start`