How can I solve 403:Forbidden whilst using a docker container?

Error is coming from Apache which tries to show you the directory contents as there is no index file available. Either your docker mapping is not working correctly, or your apache does not have php support installed on it. You are accessing http://localhost, try http://localhost/index.php.

If you get same error, problem is with mapping. If you get php code the problem is with missing PHP support in Apache.