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

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

@@ -0,0 +1,37 @@
+# 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
+}