Config fix for #2208 : Document network interface fix for multi-network configuations (#2555)

* document fix for #2208

* typo fix

* prettier

* fix code block format

* fix indentation

---------

Co-authored-by: Timothy Pillow <timothy.pillow@swisscom.com>
Co-authored-by: Bernd Storath <999999bst@gmail.com>
Co-authored-by: Bernd Storath <32197462+kaaax0815@users.noreply.github.com>

docs/content/faq.md

@@ -109,3 +109,31 @@ To resolve this issue, you can try the following steps:
     ```shell
      echo "ip6table_filter" | sudo tee -a /etc/modules
     ```
+
+## Clients lose connectivity after restarting the container when using multiple networks?
+
+When you attach multiple Docker networks (e.g., `wg` and a reverse proxy network like `traefik` or `nginx`) to the `wg-easy` container, Docker might assign the network interfaces randomly (e.g., swapping `eth0` and `eth1`). Since `wg-easy` expects the wireguard interface to act as `eth0` and configures `POSTROUTING` rules for it, connectivity will break if the interfaces are swapped upon container restart.
+
+To solve this, specify the `interface_name` and `gw_priority` explicitly in your `docker-compose.yml` file to guarantee that the `wg` network always binds to `eth0` and acts as the default gateway.
+
+**Example `docker-compose.yml`:**
+
+```yaml
+services:
+    wg-easy:
+        # ... other configuration ...
+        networks:
+            wg:
+                interface_name: eth0
+                gw_priority: 1
+                ipv4_address: 10.42.42.42
+            nginx:
+                interface_name: eth1
+                gw_priority: 0
+
+networks:
+    wg:
+        # ... wg network config ...
+    nginx:
+        external: true
+```