diff --git a/docker/Dockerfile b/docker/Dockerfile
index 83e5877..96f5d8d 100644
--- a/docker/Dockerfile
+++ b/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