docs(README): add network interface binding documentation (#1008)

Signed-off-by: Mohsen Rezaei <[email protected]>
Co-authored-by: favonia <[email protected]>

CONTRIBUTORS.markdown

@@ -3,6 +3,7 @@
 - Favonia @favonia contributed the majority of the code and its documentation.
 - Brandon @skarekrow provided a sample OpenBSD `rc.d` script.
 - @JvdMaat helped improve [`README`](./README.markdown).
+- @mohsenrezaeithe contributed to improvements and documentation.
 
 # Past Contributors
 

README.markdown

@@ -140,6 +140,13 @@ services:
         # Your domains (separated by commas)
       - PROXIED=true
         # Tell Cloudflare to cache webpages and hide your IP (optional)
+#networks:
+#  LAN0:
+#    external: true
+#    name: LAN0
+# Introduce custom Docker networks to the 'services' in this file. A common use case
+# for this is binding one of the 'services' to a specific network interface available at
+# Docker's host. This section is required for the 'networks' section of each 'services'.
 ```
 
 <details>
@@ -179,6 +186,35 @@ The easiest way to enable IPv6 is to use `network_mode: host` so that the update
 
 </details>
 
+<details>
+<summary><em>Click to expand:</em> 🛜 Bind to a specific network interface for updates</summary>
+
+📜 This method uses a MacVLAN sub-device to bind to a specific network interface and may bypass your custom `iptables` and `nftables` configurations.
+
+To be able to use a specific network interface when detecting the IP in the DDNS updates, the following Docker network must be created before running a Docker container with a custom network:
+
+```bash
+docker network create
+    -d macvlan
+    -o parent=eth0 # host network interface name to bind to
+    --subnet=192.168.1.0/24 # IP space for running containers within this network
+    --gateway=192.168.1.1 # IP address of the gateway/router
+    --ip-range=192.168.1.128/25 # communication IP range for containers in this network
+    LAN0 # name that will be used in the docker-compose.yml
+```
+
+Once the new Docker network is created, add the following to the Docker Compose that will start the `cloudflare-ddns` service. This enforces all requests from this service to go through the mentioned network, e.g. 'LAN0'.
+
+```yaml
+networks:
+  LAN0:
+  #  ipv4_address: 192.168.1.131 # A static IP within subnet (line can be removed for a random IP)
+```
+
+If a static IP is preferred, an `ipv4_address` section like the example can be added. NOTE: this IP must be within the `--subnet` of the Docker network.
+
+</details>
+
 <details>
 <summary><em>Click to expand:</em> 🛡️ Change <code>user: "1000:1000"</code> to the user and group IDs you want to use</summary>