Set a fixed ERLANG_NODE, document how to change it

Mention in the next ejabberd release notes that the default ERLANG_NODE
changed, and existing mnesia databases require setting the old one manually

ecs/README.md

@@ -216,6 +216,31 @@ Example usage (see the full [docker-compose.yml](https://github.com/processone/d
                      status
 ```
 
+## Clustering
+
+When setting several Docker containers to form a
+[cluster of ejabberd nodes](https://docs.ejabberd.im/admin/guide/clustering/),
+each one must have a different
+[Erlang Node Name](https://docs.ejabberd.im/admin/guide/security/#erlang-node-name)
+and the same
+[Erlang Cookie](https://docs.ejabberd.im/admin/guide/security/#erlang-cookie).
+For this you can either:
+- edit `conf/ejabberdctl.cfg` and set variables `ERLANG_NODE` and `ERLANG_COOKIE`
+- set the environment variables `ERLANG_NODE_ARG` and `ERLANG_COOKIE`
+
+Once you have the ejabberd nodes properly set and running,
+you can tell the secondary nodes to join the master node using the
+[join_cluster](https://docs.ejabberd.im/developer/ejabberd-api/admin-api/#join-cluster)
+API call.
+
+Example using environment variables (see the full [docker-compose.yml](https://github.com/processone/docker-ejabberd/issues/64#issuecomment-887741332) example):
+```yaml
+environment:
+  - ERLANG_NODE_ARG=ejabberd@replica1
+  - ERLANG_COOKIE=dummycookie123
+  - CTL_ON_CREATE=join_cluster ejabberd@main
+```
+
 # Generating ejabberd release
 
 ## Configuration

ecs/bin/ejabberdctl

@@ -8,7 +8,7 @@ ERL_PROCESSES=250000
 ERL_MAX_ETS_TABLES=1400
 FIREWALL_WINDOW="4370-4379"
 INET_DIST_INTERFACE=""
-ERLANG_NODE=ejabberd@$(hostname -s)
+ERLANG_NODE=ejabberd@localhost
 EJABBERD_BYPASS_WARNINGS=true
 
 # define default environment variables

ecs/conf/ejabberdctl.cfg

@@ -130,7 +130,7 @@
 #
 # Default: ejabberd@localhost
 #
-#ERLANG_NODE=ejabberd@$(hostname)
+#ERLANG_NODE=ejabberd@$(hostname -s)
 
 #.
 #' ERLANG_COOKIE: Erlang cookie for inter-node communication