Change configuration and docker compose

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/mobilizon_web/endpoint.ex

@@ -66,7 +66,10 @@ defmodule MobilizonWeb.Endpoint do
   """
   def init(_key, config) do
     if config[:load_from_system_env] do
-      port = System.get_env("PORT") || raise "expected the PORT environment variable to be set"
+      port =
+        System.get_env("MOBILIZON_INSTANCE_PORT") ||
+          raise "expected the MOBILIZON_INSTANCE_PORT environment variable to be set"
+
       {:ok, Keyword.put(config, :http, [:inet6, port: port])}
     else
       {:ok, config}