Use bbb-conf to get BBB-URL instead of `hostname -f`.

debian/postinst

 #!/bin/bash -ve
 
 # get api secret
-BBB_API_URL=https://$(hostname -f)/bigbluebutton/
+BBB_API_URL=$(bbb-conf --secret | grep URL | head -n1 | awk '{print $2}')
 BBB_API_SECRET=$(bbb-conf --secret | grep Secret | head -n1 | awk '{print $2}')
 ENV_FILE=/etc/cleanup-meeting-data/.env