Running tests in docker

Silverstripe Version: 4.3

Question:

I am wanting to run unit tests on my module.
Within my docker container I run

vendor/bin/phpunit app/tests 

And it gives the following error

Fatal error: Class 'SilverStripe\Dev\SapphireTest' not found in /var/www/html/app/tests/Model/MembershipTest.php on line 12

Call Stack:
    0.2025     397440   1. {main}() /var/www/html/vendor/phpunit/phpunit/phpunit:0
    0.2962    1509824   2. PHPUnit\TextUI\Command::main() /var/www/html/vendor/phpunit/phpunit/phpunit:61
    0.2962    1509936   3. PHPUnit\TextUI\Command->run() /var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:162
    0.3257    2196584   4. PHPUnit\TextUI\TestRunner->getTest() /var/www/html/vendor/phpunit/phpunit/src/TextUI/Command.php:183
    0.3367    2352592   5. PHPUnit\Framework\TestSuite->addTestFiles() /var/www/html/vendor/phpunit/phpunit/src/Runner/BaseTestRunner.php:70
    0.3367    2352592   6. PHPUnit\Framework\TestSuite->addTestFile() /var/www/html/vendor/phpunit/phpunit/src/Framework/TestSuite.php:618
    0.3367    2352592   7. PHPUnit\Util\FileLoader::checkAndLoad() /var/www/html/vendor/phpunit/phpunit/src/Framework/TestSuite.php:540
    0.3373    2352800   8. PHPUnit\Util\FileLoader::load() /var/www/html/vendor/phpunit/phpunit/src/Util/FileLoader.php:45
    0.3387    2356144   9. include_once('/var/www/html/app/tests/Model/MembershipTest.php') /var/www/html/vendor/phpunit/phpunit/src/Util/FileLoader.php:57