Add usage with Tunnelmole to docs (#2881)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Anbraten <anton@ju60.de>

docs/docs/30-administration/70-proxy.md

@@ -98,9 +98,31 @@ woodpeckeragent.example.com {
 Above configuration shows how to create reverse-proxies for web and agent communication. If your agent uses SSL do not forget to enable [WOODPECKER_GRPC_SECURE](./15-agent-config.md#woodpecker_grpc_secure).
 :::
 
+## Tunnelmole
+
+[Tunnelmole](https://github.com/robbie-cahill/tunnelmole-client) is an open source tunneling tool.
+
+Start by [installing tunnelmole](https://github.com/robbie-cahill/tunnelmole-client#installation).
+
+After the installation, run the following command to start tunnelmole:
+
+```bash
+tmole 8000
+```
+
+It will start a tunnel and will give a response like this:
+
+```bash
+➜  ~ tmole 8000
+http://bvdo5f-ip-49-183-170-144.tunnelmole.net is forwarding to localhost:8000
+https://bvdo5f-ip-49-183-170-144.tunnelmole.net is forwarding to localhost:8000
+```
+
+Set `WOODPECKER_HOST` (for example in `docker-compose.yml`) to the Tunnelmole URL (`xxx.tunnelmole.net`) and start the server.
+
 ## Ngrok
 
-After installing [ngrok](https://ngrok.com/), open a new console and run:
+[Ngrok](https://ngrok.com/) is a popular closed source tunnelling tool. After installing ngrok, open a new console and run the following command:
 
 ```bash
 ngrok http 8000