There is a curious circular dependency issue in Contao 4.13 when you try to decorate (or override) the `router` with your own `ChainRouter`. The first built container example shows the `getRouterService()` method without the decoration, the second with.

a_Contao_ManagerBundle_HttpKernel_ContaoKernelProdContainer.php

<?php

/**
 * Here we have
 *   $this->services['router'] = $instance = new \Symfony\Cmf\Component\Routing\ChainRouter($a);
 * almost right at the start, defining the `router` service very early.
 */
protected function getRouterService()
{
    $a = ($this->privates['monolog.logger'] ?? $this->getMonolog_LoggerService());

    $this->services['router'] = $instance = new \Symfony\Cmf\Component\Routing\ChainRouter($a);

    $b = ($this->privates['router.request_context'] ?? $this->getRouter_RequestContextService());
    $c = new \Symfony\Bridge\Monolog\Logger('router');
    $c->pushProcessor(($this->privates['contao.monolog.processor'] ?? $this->getContao_Monolog_ProcessorService()));
    $c->pushHandler(($this->privates['monolog.handler.console'] ?? $this->getMonolog_Handler_ConsoleService()));
    $c->pushHandler(($this->privates['monolog.handler.main'] ?? $this->getMonolog_Handler_MainService()));
    $c->pushHandler(($this->privates['contao.monolog.handler'] ?? $this->getContao_Monolog_HandlerService()));

    $d = new \Symfony\Bundle\FrameworkBundle\Routing\Router((new \Symfony\Component\DependencyInjection\Argument\ServiceLocator($this->getService, [
        'routing.loader' => ['services', 'routing.loader', 'getRouting_LoaderService', true],
    ], [
        'routing.loader' => 'Symfony\\Component\\Config\\Loader\\LoaderInterface',
    ]))->withContext('router.default', $this), 'contao_manager.routing.route_loader::loadFromPlugins', ['cache_dir' => $this->targetDir.'', 'debug' => false, 'generator_class' => 'Symfony\\Component\\Routing\\Generator\\CompiledUrlGenerator', 'generator_dumper_class' => 'Symfony\\Component\\Routing\\Generator\\Dumper\\CompiledUrlGeneratorDumper', 'matcher_class' => 'Symfony\\Bundle\\FrameworkBundle\\Routing\\RedirectableCompiledUrlMatcher', 'matcher_dumper_class' => 'Symfony\\Component\\Routing\\Matcher\\Dumper\\CompiledUrlMatcherDumper', 'strict_requirements' => false, 'resource_type' => 'service'], $b, ($this->privates['parameter_bag'] ?? ($this->privates['parameter_bag'] = new \Symfony\Component\DependencyInjection\ParameterBag\ContainerBag($this))), $c, 'en');
    $d->setConfigCacheFactory(($this->privates['config_cache_factory'] ?? ($this->privates['config_cache_factory'] = new \Symfony\Component\Config\ResourceCheckerConfigCacheFactory())));
    $d->addExpressionLanguageProvider(($this->privates['router.expression_language_provider'] ?? $this->getRouter_ExpressionLanguageProviderService()));
    $e = ($this->privates['contao.routing.route_provider'] ?? $this->getContao_Routing_RouteProviderService());

    $f = new \Contao\CoreBundle\Routing\PageUrlGenerator($e, ($this->services['contao.routing.page_registry'] ?? $this->getContao_Routing_PageRegistryService()), $a);
    $g = ($this->services['event_dispatcher'] ?? $this->getEventDispatcherService());
    $h = new \Symfony\Cmf\Component\Routing\DynamicRouter($b, ($this->services['contao.routing.nested_matcher'] ?? $this->getContao_Routing_NestedMatcherService()), $f, '', $g, $e);
    $h->addRouteEnhancer(new \Contao\CoreBundle\Routing\Enhancer\InputEnhancer(($this->services['contao.framework'] ?? $this->getContao_FrameworkService())), 0);

    $instance->setContext($b);
    $instance->add($d, '100');
    $instance->add(new \Symfony\Cmf\Component\Routing\DynamicRouter($b, ($this->services['contao.routing.nested_404_matcher'] ?? $this->getContao_Routing_Nested404MatcherService()), $f, '', $g, ($this->privates['contao.routing.route_404_provider'] ?? $this->getContao_Routing_Route404ProviderService())), -200);
    $instance->add($h, 20);

    return $instance;
}

b_Contao_ManagerBundle_HttpKernel_ContaoKernelProdContainer.php

<?php

/**
 * Here we have
 *   $this->services['router'] = $instance = new \App\Routing\ChainRouter($h);
 * much later, which causes a recursion between `@contao.framework` and `@contao.routing.route_provider`.
 * `@contao.framework` tries to get the `@router`, `getRouterService()` tries to get `@contao.routing.route_provider`
 * wich then tries to get `@contao.framework`, which then tries to get the `@router` etc.
 */
protected function getRouterService()
{
    $a = ($this->privates['contao.routing.route_provider'] ?? $this->getContao_Routing_RouteProviderService());

    if (isset($this->services['router'])) {
        return $this->services['router'];
    }
    $b = ($this->services['contao.routing.page_registry'] ?? $this->getContao_Routing_PageRegistryService());

    if (isset($this->services['router'])) {
        return $this->services['router'];
    }
    $c = ($this->services['event_dispatcher'] ?? $this->getEventDispatcherService());

    if (isset($this->services['router'])) {
        return $this->services['router'];
    }
    $d = ($this->privates['contao.routing.route_404_provider'] ?? $this->getContao_Routing_Route404ProviderService());

    if (isset($this->services['router'])) {
        return $this->services['router'];
    }
    $e = ($this->services['contao.routing.nested_matcher'] ?? $this->getContao_Routing_NestedMatcherService());

    if (isset($this->services['router'])) {
        return $this->services['router'];
    }
    $f = ($this->services['contao.framework'] ?? $this->getContao_FrameworkService());

    if (isset($this->services['router'])) {
        return $this->services['router'];
    }
    $g = ($this->privates['monolog.logger'] ?? $this->getMonolog_LoggerService());

    $h = new \Symfony\Cmf\Component\Routing\ChainRouter($g);

    $i = ($this->privates['router.request_context'] ?? $this->getRouter_RequestContextService());
    $j = new \Symfony\Bridge\Monolog\Logger('router');
    $j->pushProcessor(($this->privates['contao.monolog.processor'] ?? $this->getContao_Monolog_ProcessorService()));
    $j->pushHandler(($this->privates['monolog.handler.console'] ?? $this->getMonolog_Handler_ConsoleService()));
    $j->pushHandler(($this->privates['monolog.handler.main'] ?? $this->getMonolog_Handler_MainService()));
    $j->pushHandler(($this->privates['contao.monolog.handler'] ?? $this->getContao_Monolog_HandlerService()));

    $k = new \Symfony\Bundle\FrameworkBundle\Routing\Router((new \Symfony\Component\DependencyInjection\Argument\ServiceLocator($this->getService, [
        'routing.loader' => ['services', 'routing.loader', 'getRouting_LoaderService', true],
    ], [
        'routing.loader' => 'Symfony\\Component\\Config\\Loader\\LoaderInterface',
    ]))->withContext('router.default', $this), 'contao_manager.routing.route_loader::loadFromPlugins', ['cache_dir' => $this->targetDir.'', 'debug' => false, 'generator_class' => 'Symfony\\Component\\Routing\\Generator\\CompiledUrlGenerator', 'generator_dumper_class' => 'Symfony\\Component\\Routing\\Generator\\Dumper\\CompiledUrlGeneratorDumper', 'matcher_class' => 'Symfony\\Bundle\\FrameworkBundle\\Routing\\RedirectableCompiledUrlMatcher', 'matcher_dumper_class' => 'Symfony\\Component\\Routing\\Matcher\\Dumper\\CompiledUrlMatcherDumper', 'strict_requirements' => false, 'resource_type' => 'service'], $i, ($this->privates['parameter_bag'] ?? ($this->privates['parameter_bag'] = new \Symfony\Component\DependencyInjection\ParameterBag\ContainerBag($this))), $j, 'en');
    $k->setConfigCacheFactory(($this->privates['config_cache_factory'] ?? ($this->privates['config_cache_factory'] = new \Symfony\Component\Config\ResourceCheckerConfigCacheFactory())));
    $k->addExpressionLanguageProvider(($this->privates['router.expression_language_provider'] ?? $this->getRouter_ExpressionLanguageProviderService()));
    $this->services['router'] = $instance = new \App\Routing\ChainRouter($h);
    $l = new \Contao\CoreBundle\Routing\PageUrlGenerator($a, $b, $g);
    $m = new \Symfony\Cmf\Component\Routing\DynamicRouter($i, $e, $l, '', $c, $a);
    $m->addRouteEnhancer(new \Contao\CoreBundle\Routing\Enhancer\InputEnhancer($f), 0);

    $h->setContext($i);
    $h->add($k, '100');
    $h->add(new \Symfony\Cmf\Component\Routing\DynamicRouter($i, ($this->services['contao.routing.nested_404_matcher'] ?? $this->getContao_Routing_Nested404MatcherService()), $l, '', $c, $d), -200);
    $h->add($m, 20);

    return $instance;
}

ChainRouter.php

<?php

namespace App\Routing;

use Symfony\Cmf\Component\Routing\ChainRouterInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\CacheWarmer\WarmableInterface;
use Symfony\Component\Routing\RequestContext;

class ChainRouter implements ChainRouterInterface, WarmableInterface
{
    public function __construct(private readonly ChainRouterInterface&WarmableInterface $inner)
    {
    }

    public function add($router, $priority = 0): void
    {
        $this->inner->add($router, $priority);
    }

    public function all()
    {
        return $this->inner->all();
    }

    public function warmUp(string $cacheDir, string|null $buildDir = null): void
    {
        $this->inner->warmUp($cacheDir);
    }

    public function getRouteCollection()
    {
        return $this->inner->getRouteCollection();
    }

    public function match(string $pathinfo)
    {
        return $this->inner->match($pathinfo);
    }

    public function matchRequest(Request $request)
    {
        return $this->inner->matchRequest($request);
    }

    public function generate(string $name, array $parameters = [], int $referenceType = self::ABSOLUTE_PATH): string
    {
        return $this->inner->generate($name, $parameters, $referenceType);
    }

    public function setContext(RequestContext $context): void
    {
        $this->inner->setContext($context);
    }

    public function getContext(): RequestContext
    {
        return $this->inner->getContext();
    }
}

services.yaml

services:
    App\ChainRouter:
        decorates: cmf_routing.router
        arguments: ['@.inner']