Today's `docker-compose` docs are missing the `discovery.seed_nodes` config on
one of the nodes. With today's configuration the cluster can still form the
first time it is started, because `cluster.initial_master_nodes` requires both
nodes to bootstrap the cluster which ensures that each discover the other.
However if `es02` is elected master it will remove `es01` from the voting
configuration and then when restarted it will form a cluster on its own without
needing to do any discovery. Meanwhile `es01` doesn't know how to find `es02`
after a restart so will be unable to join this cluster.
This commit fixes this by adding the missing configuration.
b6b98f72ce