Use dynamic docker

Author: Greg Bowler

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: php-actions/composer@master
       - name: PHPUnit Tests
-        uses: php-actions/phpunit@17-php-versions
+        uses: php-actions/phpunit@dynamic-docker
         with:
           bootstrap: vendor/autoload.php
           configuration: test/phpunit.xml