feat: update the Docker command in the docs

as per the suggestion
netbirdio/netbird#2290 (comment)

Author: nazarewk

src/pages/how-to/examples.mdx

@@ -104,10 +104,16 @@ You would need to obtain a [setup key](/how-to/register-machines-using-setup-key
 
 The setup key could be found in the NetBird Management dashboard under the Setup Keys tab - [https://app.netbird.io/setup-keys](https://app.netbird.io/setup-keys).
 
-Set the ```NB_SETUP_KEY``` environment variable and run the command.
+Set the environment variables:
+
+- `PEER_NAME` - Peer's name and hostname as appearing in the Dashboard/DNS queries
+- `NB_SETUP_KEY`
+- `EXTERNAL_INTERFACE` - an external interface, the machine uses for Internet access
+
+and run the command:
 
 ```bash
-docker run --rm --name PEER_NAME --hostname PEER_NAME --cap-add=NET_ADMIN --cap-add=SYS_ADMIN --cap-add=SYS_RESOURCE -d -e NB_SETUP_KEY=<SETUP KEY> -v netbird-client:/etc/netbird netbirdio/netbird:latest
+docker run --rm -d --name "$PEER_NAME" --hostname "$PEER_NAME" --cap-add=NET_ADMIN --cap-add=SYS_ADMIN --cap-add=SYS_RESOURCE -e NB_SETUP_KEY="$NB_SETUP_KEY" -v netbird-client:"/etc/netbird" --sysctl "net.ipv4.conf.all.src_valid_mark=1" --sysctl "net.ipv4.conf.all.rp_filter=2" --sysctl "net.ipv4.conf.${EXTERNAL_INTERFACE}.rp_filter=2" netbirdio/netbird:latest
 ```
 
 That is it! Enjoy using NetBird.