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

docker/examples/docker-compose-cluster/Dockerfile

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+FROM rroemhild/ejabberd
+ENV EJABBERD_HOME /opt/ejabberd
+COPY ./scripts $EJABBERD_HOME/scripts

docker/examples/docker-compose-cluster/README.md

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+# Ejabberd cluster with docker compose
+
+This example uses [dnsdocker](https://github.com/tonistiigi/dnsdock) to discover other nodes and setup a multi-master cluster.
+
+Build the ejabberd cluster image:
+
+```bash
+git clone https://github.com/rroemhild/docker-ejabberd.git
+cd docker-ejabberd/examples/docker-compose-cluster
+docker-compose build
+```
+
+Start dnsdocker and the first ejabberd node:
+
+```bash
+docker-compose up -d
+```
+
+Wait until the first ejabberd node is up and running `docker-compose logs ejabberd`, then add some ejabberd nodes to the cluster:
+
+```bash
+docker-compose scale ejabberd=4
+```

docker/examples/docker-compose-cluster/docker-compose.yml

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+dnsdock:
+  image: tonistiigi/dnsdock
+  volumes:
+    - /var/run/docker.sock:/var/run/docker.sock
+  ports:
+    - 172.17.42.1:53:53/udp
+
+ejabberd:
+  build: .
+  ports:
+    - 5222
+    - 5269
+    - 5280
+  environment:
+    - XMPP_DOMAIN=example.com
+    - ERLANG_NODE=ejabberd
+    - EJABBERD_ADMINS=admin@example.com
+    - EJABBERD_USERS=admin@example.com:test321 user@example.com
+    - ERLANG_COOKIE=testCluster
+    - SKIP_MODULES_UPDATE=true
+    - EJABBERD_CLUSTER=true
+    - USE_DNS=true
+  dns: 172.17.42.1
+  domainname: dockercomposecluster_ejabberd.docker
+  tty: true

docker/examples/docker-compose-cluster/scripts/lib/functions.sh

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+# overwrite get_nodename to discover hostname from DNS
+get_nodename() {
+    local hostname=${HOSTNAME}
+
+    # get hostname from dns
+    if ( is_true ${USE_DNS} ); then
+        # wait for dns registration
+        sleep 1
+
+        nodename=$(discover_dns_hostname ${HOSTIP})
+
+        is_set ${nodename} \
+            && hostname=${nodename}
+    fi
+
+    echo $hostname
+    return 0
+}
+
+
+# discover hostname from dns with a reverse lookup
+discover_dns_hostname() {
+    local hostip=$1
+
+    # try to get the hostname from dns
+    local dnsname=$(drill -x ${hostip} \
+        | grep PTR \
+        | awk '{print $5}' \
+        | grep -E "^[a-zA-Z0-9]+([-._]?[a-zA-Z0-9]+)*.[a-zA-Z]+\.$" \
+        | cut -d '.' -f1 \
+        | tail -1)
+
+    is_set ${dnsname} \
+        && echo ${dnsname}
+
+    return 0
+}

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

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+#!/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