Add debug commands

docs/content/advanced/mail-fetchmail.md

@@ -30,4 +30,13 @@ poll pop3.example.com with proto POP3
 	here options keep ssl
 ```
 
+To debug your `fetchmail.cf` configuration run this command:
+
+```
+docker run --rm \
+  -v "$(pwd)/config:/tmp/docker-mailserver" \
+  -ti tvial/docker-mailserver:latest \
+  sh -c "cat /etc/fetchmailrc_general /tmp/docker-mailserver/fetchmail.cf > /etc/fetchmailrc; /etc/init.d/fetchmail debug-run"
+```
+
 By default the fetchmail service searches very 5 minutes for new mails on your external mail accounts.