Ok, this is overkill, but HOCON substitutions are nice.

hocon-with-replacements.conf

akka {
  loggers = [
    "akka.event.slf4j.Slf4jLogger"
  ]
  loglevel = warning
  actor {
    provider = "akka.cluster.ClusterActorRefProvider"
  }
  log-dead-letters = 5
  log-dead-letters-during-shutdown = off

  remote {
    netty.tcp {
      hostname = ${primary-cluster.local.ip}
      port = ${primary-cluster.local.port}
    }
  }
  
  cluster {
    seed-nodes = [
       "akka.tcp://"${primary-cluster.name}"@"${primary-cluster.seed.ip}":"${primary-cluster.seed.port}
    ]
    auto-down-unreachable-after = 10s
  }
}

primary-cluster {
  name = provisioning-pool

  loopback-ip   = "127.0.0.1"
  default-port  = 19123

  # The local node's configuration
  local {
    # Loopback is only used when debugging...
    ip = ${primary-cluster.loopback-ip}

    # ...otherwise, get the cluster IP from an environment variable.
    ip = ${?LOCAL_CLUSTER_IP}

    # Our default port.
    port = ${primary-cluster.default-port}

    # ...but allow override via environment variable.
    port = ${?LOCAL_CLUSTER_PORT}
  }

  # Where to find the cluster seed node.
  seed {
    # Loopback is only used when debugging...
    ip = ${primary-cluster.loopback-ip}

    # ...otherwise, if we're the seed node, use the cluster seed IP...
    ip = ${?LOCAL_CLUSTER_IP}

    # ...but, if we're not the seed node, try to get the seed node's IP address from the environment variable set by Docker's container link.
    ip = ${?SEED_PORT_19123_TCP_ADDR}

    # Our default port.
    port = ${primary-cluster.default-port}

    # ...but, if we're not the seed node, try to get the seed node's port from the environment variable set by Docker's container link.
    port = ${?SEED_PORT_19123_TCP_PORT}
  }
}