Add support for $RABBITMQ_ERLANG_COOKIE env var

It is the equivalent of the content of the Erlang cookie file. Note this
variable IS the cookie value, NOT the path to a cookie file.

If it is set, it will take precedence over the content of the Erlang
cookie file.

Fixes docker-library/rabbitmq#508.

deps/rabbit/apps/rabbitmq_prelaunch/src/rabbit_prelaunch_dist.erl

@@ -1,5 +1,6 @@
 -module(rabbit_prelaunch_dist).
 
+-include_lib("eunit/include/eunit.hrl").
 -include_lib("kernel/include/logger.hrl").
 
 -include_lib("rabbit_common/include/logging.hrl").
@@ -33,7 +34,9 @@ setup(#{nodename := Node, nodename_type := NameType} = Context) ->
     end,
     ok.
 
-do_setup(#{nodename := Node, nodename_type := NameType}) ->
+do_setup(#{nodename := Node,
+           nodename_type := NameType,
+           var_origins := Origins} = Config) ->
     ?LOG_DEBUG(
        "Starting Erlang distribution",
        #{domain => ?RMQLOG_DOMAIN_PRELAUNCH}),
@@ -49,6 +52,19 @@ do_setup(#{nodename := Node, nodename_type := NameType}) ->
             {ok, _} = net_kernel:start([Node, NameType]),
             ok
     end,
+
+    %% Override the Erlang cookie if one was set in the environment.
+    case maps:get(erlang_cookie, Origins, default) of
+        environment ->
+            ?LOG_DEBUG(
+               "Override Erlang cookie using the one set in the environment",
+               #{domain => ?RMQLOG_DOMAIN_PRELAUNCH}),
+            Cookie = maps:get(erlang_cookie, Config),
+            ?assert(is_atom(Cookie)),
+            true = erlang:set_cookie(node(), Cookie);
+        _ ->
+            ok
+    end,
     ok.
 
 %% Check whether a node with the same name is already running