It's possible to bootstrap Drupal 8 and create a new node using answer # 174474.

define('DRUPAL_DIR', '/usr/share/nginx/html');
use Drupal\Core\DrupalKernel;
use Symfony\Component\HttpFoundation\Request;

require_once DRUPAL_DIR . '/core/includes/database.inc';
require_once DRUPAL_DIR . '/core/includes/schema.inc';

// Specify relative path to the drupal root.
$autoloader = require_once DRUPAL_DIR . '/autoload.php';
$request = Request::createFromGlobals();

// Bootstrap drupal to different levels
$kernel = DrupalKernel::createFromRequest($request, $autoloader, 'prod');
$kernel->boot();
$kernel->prepareLegacyRequest($request);
$em = $kernel->getContainer()->get('entity.manager');

$entity = $em->getStorage('node')->create(
    array(
      'type' => "article",
      'title'=> "test entity",
      'body' => "body body body",
    ));

$entity->save();

Is it possible to bootstrap a subsite database when using multisite to create the new node in a specific subsite?

In Drupal 7, this can be done by using the function drupal_override_server_variables, but this function doesn't exist in Drupal 8.

I also tried modifying the $_SERVER['HTTP_HOST'] and $_SERVER['SCRIPT_NAME'] variables according to this forum post, with no luck.