Don't allocate tty

Removing the -t option makes the backup script executable by cron and similar tools.

docs/u_e-backup_restore-maildir.md

@@ -5,7 +5,7 @@ This line backups the vmail directory to a file backup_vmail.tar.gz in the mailc
 cd /path/to/mailcow-dockerized
 source mailcow.conf
 DATE=$(date +"%Y%m%d_%H%M%S")
-docker run --rm -it -v $(docker inspect --format '{{ range .Mounts }}{{ if eq .Destination "/var/vmail" }}{{ .Name }}{{ end }}{{ end }}' $(docker-compose ps -q dovecot-mailcow)):/vmail -v ${PWD}:/backup debian:jessie tar cvfz /backup/backup_vmail.tar.gz /vmail
+docker run --rm -i -v $(docker inspect --format '{{ range .Mounts }}{{ if eq .Destination "/var/vmail" }}{{ .Name }}{{ end }}{{ end }}' $(docker-compose ps -q dovecot-mailcow)):/vmail -v ${PWD}:/backup debian:jessie tar cvfz /backup/backup_vmail.tar.gz /vmail
 ```
 
 You can change the path by adjusting ${PWD} (which equals to the current directory) to any path you have write-access to.