fixed tests, added php 8 to dockerfile

Author: alxmrz

composer.json

@@ -2,7 +2,7 @@
   "name": "ff/ff-core",
   "require": {
     "monolog/monolog": "1.0.*",
-    "phpunit/phpunit": "^6.5",
+    "phpunit/phpunit": "^9",
     "twig/twig": "^2.0",
     "psr/log": "^1.0",
     "squizlabs/php_codesniffer": "*",