Fix TestCase::getContainer method signature

Author: mcg-web

tests/Functional/TestCase.php

@@ -9,6 +9,7 @@
 use Symfony\Bundle\FrameworkBundle\Client;
 use Symfony\Bundle\FrameworkBundle\KernelBrowser;
 use Symfony\Bundle\FrameworkBundle\Test\WebTestCase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\Filesystem\Filesystem;
 use Symfony\Component\HttpFoundation\Request;
 
@@ -90,7 +91,7 @@ protected static function assertGraphQL(string $query, array $expectedData = nul
         static::assertSame($expected, $result, \json_encode($result));
     }
 
-    protected static function getContainer()
+    protected static function getContainer(): ContainerInterface
     {
         return static::$kernel->getContainer();
     }