symfony6 $this->container->get('app.our.useful.thing') - service

Question

I am sorry, I read the docs, but I don't get symfony6 anymore.
services:
# default configuration for services in *this* file
_defaults:
autowire: true # Automatically injects dependencies in your services.
autoconfigure: true # Automatically registers your services as commands, event subscribers, etc.
# makes classes in src/ available to be used as services
# this creates a service per class whose id is the fully-qualified class name
App\:
resource: '../src/'
exclude:
- '../src/DependencyInjection/'
- '../src/Entity/'
- '../src/Kernel.php'
- '../src/EventListener
app.our.useful.thing:
class: App\OurUsefulThing
public: true
My src/OurUsefulThing.php:
<?php
namespace App;
class OurUsefulThing
{
public function sayHello()
{
return "Hello Service";
}
}
In the controller:
public function someActionName(Request $request)() {
$value = $this->container->get('app.our.useful.thing')->sayHello();
}
Error Message
Service "app.our.useful.thing" not found: even though it exists in the app's container, the container inside "App\Controller\CategoriesController" is a smaller service locator that only knows about the "form.factory", "http_kernel", "parameter_bag", "request_stack", "router", "security.authorization_checker", "security.csrf.token_manager", "security.token_storage", "serializer" and "twig" services. Try using dependency injection instead.
It was in symfony4 so easy. What I am doing wrong?