Use deploy.sh to start containers. You can connect to any node:
docker run -it node2 mysql
To add new node (connected to any up node)
docker run --detach=true --name node7 -h node7 --link node2:node2 who0/docker-galera --wsrep-cluster-name=local-test --wsrep-cluster-address=gcomm://node2
Based on Galera doc