Drupal - How to get the base URL of a site

You can get the hostname, "drupal8.local", directly from the getHost() request:

$host = \Drupal::request()->getHost();

In some cases you might want to get the schema as well, fx https://drupal8.local:

$host = \Drupal::request()->getSchemeAndHttpHost();

There are some warnings about directly accessing the request object in this way in \Drupal::request:

 * Note: The use of this wrapper in particular is especially discouraged. Most
 * code should not need to access the request directly.  Doing so means it
 * will only function when handling an HTTP request, and will require special
 * modification or wrapping when run from a command line tool, from certain
 * queue processors, or from automated tests.
 *
 * If code must access the request, it is considerably better to register
 * an object with the Service Container and give it a setRequest() method
 * that is configured to run when the service is created.  That way, the
 * correct request object can always be provided by the container and the
 * service can still be unit tested.

Any form controller extending \Drupal\Core\Form\FormBase automatically has this dependency injected, and it may be accessed using:

$this->getRequest()->getSchemeAndHttpHost()

I think (but haven't tested) that a regular page controller extending \Drupal\Core\Controller\ControllerBase could provide the request_stack service by overriding the \Drupal\Core\Controller\ControllerBase::create function, and then setting a $request property in the constructor. This is described really well for forms, and the same process should apply for page controllers: https://www.drupal.org/docs/8/api/services-and-dependency-injection/dependency-injection-for-a-form.

If you'd like to do this with dependency injection and a service, then you can use RequestStack:

use Symfony\Component\HttpFoundation\RequestStack;

And define it like this:

protected $request;

public function __construct(..., RequestStack $request_stack) {
  ...
  $this->request = $request_stack->getCurrentRequest();
}

public static function create(ContainerInterface $container, ...) {
  return new static(
    ...
    $container->get('request_stack')
  )
}

And then declare it like this:

$this->request->getHost()
$this->request->getSchemeAndHttpHost()