Fix discovery config in docker-compose docs (#41394)

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.

docs/reference/setup/install/docker.asciidoc

@@ -169,6 +169,7 @@ services:
     container_name: es01
     environment:
       - node.name=es01
+      - discovery.seed_hosts=es02
       - cluster.initial_master_nodes=es01,es02
       - cluster.name=docker-cluster
       - bootstrap.memory_lock=true