Add note about .env.local being ignored in test environment

Author: clemherreman

configuration.rst

@@ -627,6 +627,11 @@ the only mandatory file and each file content overrides the previous one:
   env vars overrides only for some environment. It's similar to ``.env.local``,
   but the overrides only apply to some particular environment.
 
+.. note::
+
+    ``.env.local`` is always ignored in test environment. The logic is that
+    tests should always produce the same results for everyone.
+
 .. note::
 
     The real environment variables defined in the server always win over the