Laravel gives 404 when running tests on any route when having APP_URL to anything different than http://localhost

I'm using Laravel 5.6.* . Also have tested with 5.5.*

If I set the APP_URL to anything different than http://localhost, Laravel will always return 404 when calling $this->post('someurl') or $this->get('someurl') on tests. Even the tests than come already created with Laravel.

I'm using Docker for my dev server with no special network configuration.

Everything works fine on the browser and also using postman to post to the urls.