How do I access the container using KernelTestCase and PHPUnit

My test extends Pimcore\Test\KernelTestCase but I cannot access the container.

I have read through the documentation here and have the same Bootstrap file as the example given.

I have checked the gitter channel where someone reported a similar issue, but the solution given did not work for me. When I add the following to the bootstrap.php file

$kernel = Bootstrap::kernel();
$kernel->boot();

I get the following error

Fatal error: Uncaught Symfony\Component\Config\Exception\FileLocatorFileNotFoundException: The file “./var/config/system.yml” does not exist (in: “”). in /srv/vendor/symfony/symfony/src/Symfony/Component/Config/FileLocator.php on line 71

I am using Pimcore v6.7.2

Any advice would be much appreciated, thank you!