Fix cronjob errors of Dovecot expunge

Recently, I discovered that the cronjob on my mailcow-dockerized host isn't working.
If the script is called via crontab, the docker-compose command doesn't know where the config file is located. So I added the "cd" to change the directory to the working directory of the mailcow-dockerized instance.
When executing the docker-compose exec command via cronjob, it fails with error message "the input device is not a TTY". By adding the parameter "-T" it works.

docs/u_e-dovecot-expunge.md

@@ -31,8 +31,9 @@ If you want to automate such a task you can create a cron job on your host that
 
 ```
 #!/bin/bash
-/usr/local/bin/docker-compose exec dovecot-mailcow doveadm expunge -A mailbox 'Junk' savedbefore 2w
-/usr/local/bin/docker-compose exec dovecot-mailcow doveadm expunge -A mailbox 'Junk' SEEN not SINCE 12h
+cd /opt/mailcow-dockerized
+/usr/local/bin/docker-compose exec -T dovecot-mailcow doveadm expunge -A mailbox 'Junk' savedbefore 2w
+/usr/local/bin/docker-compose exec -T dovecot-mailcow doveadm expunge -A mailbox 'Junk' SEEN not SINCE 12h
 [...]
 ```