From within the directory:
docker-compose up -d --buildVerify there are 3 nodes:
$ sudo docker exec -ti dg_node1_1 mysql -e 'show status like "wsrep_cluster_size"'
+--------------------+-------+
| Variable_name | Value |
+--------------------+-------+
| wsrep_cluster_size | 3 |
+--------------------+-------+Note this needs to be further adapted to work with nodes on different hosts, which is discussed at www.galeracluster.com [here] (http:https://galeracluster.com/2015/05/getting-started-galera-with-docker-part-2-2/).