Doc: Add setings description in docker

docs/src/passwords.md

@@ -7,3 +7,6 @@ passwords involves:
 - Create a new super user `pipenv run python manage.py createsuperuser`
 - Login to your admin panel `{your_website_url}/admin/`
 - Go to the users section, select the user, change their password.
+
+Note: If you use that command to create superuser **inside docker**, please add `--settings=pinry.settings.docker`
+to command like, just like `python manage.py createsuperuser --settings=pinry.settings.docker`