Those are transient errors. I just tried with dgraph/dgraph:latest since we released v1.0.5 and the cluster seems to work fine.
What makes you believe that the other two nodes are not stable? Are they not responding to queries/mutations?
Regarding the ELB, maybe have a look at Docker for Swarm. From the link I can see
Elastic Load Balancers (ELBs) are set up to help with routing traffic to your swarm.