The problem is obviously the backup script as well as the restore script are for a host envirnonment not for a docker install.
so the docker container zammad-backup does not have a user or group zammad:zammad
the docker container zammad-backup does not find the container of the postgresql DB
the docker container can not use the service command in order to stop the service zammad
so the backup and the restore scripts have to be adapted in order to work in docker environment
have a look at https://community.zammad.org/t/how-to-restore-zammad-backup-in-docker-compose-install/1831 , i will try also, if this approach works…