update docker-compose.test.yml to use mysql:8.1

Author: HJF9w

docker-compose.test.yml

@@ -6,7 +6,7 @@ services:
     depends_on:
     - bookstack
   mysql:
-    image: mysql:8.0
+    image: mysql:8.1
     environment:
     - MYSQL_ROOT_PASSWORD=secret
     - MYSQL_DATABASE=bookstack