Not sure why @slimphp v3 doesn't add the App instance to the Container, but…here is how to do it if you want

Application.php

<?php
namespace Core;

use Interop\Container\ContainerInterface;
use Slim\App;

class Application extends App {

    /**
     * @param ContainerInterface|array $container
     */
    public function __construct($container = [])
    {
        parent::__construct($container);
        $this->getContainer()['app'] = function () { return $this; };
    }
}