Document PHP versions

[ci skip]

Author: williamdes

docker/Dockerfile

@@ -1,4 +1,9 @@
 # syntax=docker/dockerfile:1
+# https://phpunit.de/supported-versions.html
+# PHPUnit 10 >= PHP 8.1
+# PHPUnit 9 >= PHP 7.3
+# PHPUnit 8	>= PHP 7.2
+# PHPUnit 7	PHP 7.1 - PHP 7.3
 FROM php:8.1-cli-alpine
 
 ARG RELEASE_VERSION=latest