the configuration of the cluster is not persistent. docker-entrypoint.sh creates the cluster each time the container starts
the configuration of the running cluster can be fount inside the containers in /etc/mysql/mariadb.conf.d/10-galera.cnf (docker-compose exec mariadb_node01 cat "/etc/mysql/mariadb.conf.d/10-galera.cnf")
MariaDB volumes persist until command docker-compose down -v is gived
at first startup of the cluster or when all nodes are down set BOOTSTRAP=1 on one of the nodes (--wsrep-new-cluster)