New in Symfony 3.4: Defining compiler passes in the kernel

// src/Kernel.php
namespace App;

use Symfony\Bundle\FrameworkBundle\Kernel\MicroKernelTrait;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel implements CompilerPassInterface
{
    use MicroKernelTrait;

    // ...

    public function process(ContainerBuilder $container)
    {
        // define here the code to manipulate the service container...

        // For example, change some container service:
        $container->getDefinition('app.some_private_service')->setPublic(true);

        // or process tagged services:
        foreach ($container->findTaggedServiceIds('some_tag') as $id => $tags) {
            // ...
        }
    }
}