Sync containers from rroemhild and add instructions in README (#1655)

docker/examples/docker-compose-cluster/scripts/pre/30_join_cluster.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+set -e
+
+source "${EJABBERD_HOME}/scripts/lib/base_config.sh"
+source "${EJABBERD_HOME}/scripts/lib/config.sh"
+source "${EJABBERD_HOME}/scripts/lib/base_functions.sh"
+source "${EJABBERD_HOME}/scripts/lib/functions.sh"
+
+
+get_cluster_node_from_dns() {
+    local cluster_host=$(drill ${DOMAINNAME} \
+        | grep ${DOMAINNAME} \
+        | grep -v ${HOSTIP} \
+        | awk '{print $5}' \
+        | grep -v "^$" \
+        | head -1)
+    echo $(discover_dns_hostname ${cluster_host})
+}
+
+
+file_exist ${FIRST_START_DONE_FILE} \
+    && exit 0
+
+
+join_cluster $(get_cluster_node_from_dns)
+
+
+exit 0