To add more slaves, please add one in ./docker-compose.yml and put its name in ./slaves file. Please bear in mind that newly added slave needs to mount ./flink-conf.yaml configuration (due to jobmanager.rpc.address: master entry).
Simply execute
docker-compose upfrom the project root dir and visit jobmanager gui under localhost:8081.
Branch ha contains a setup for HA standalone cluster mode. I still don't know how to run Dashboard GUI...