mailman3: fix db name in streaming file

This file streams the db backup to borg; it should be using the
"database" container from the docker-compose file.

Change-Id: Ide568215677855aa8eff1da617c4649223373052

playbooks/roles/mailman3/tasks/main.yaml

@@ -273,6 +273,6 @@
     - name: Create db streaming file
       copy:
         content: >-
-            /usr/local/bin/docker-compose -f /etc/mailman-compose/docker-compose.yaml exec -T mariadb
+            /usr/local/bin/docker-compose -f /etc/mailman-compose/docker-compose.yaml exec -T database
             bash -c '/usr/bin/mysqldump --skip-extended-insert --databases mailmandb --single-transaction -uroot -p"$MYSQL_ROOT_PASSWORD"'
         dest: /etc/borg-streams/mysql