update docker-compose.yml to use mysql:8.1

Author: HJF9w

docker-compose.yml

@@ -2,7 +2,7 @@
 version: '3'
 services:
   mysql:
-    image: mysql:8.0
+    image: mysql:8.1
     environment:
     - MYSQL_ALLOW_EMPTY_PASSWORD=1
     - MYSQL_DATABASE=bookstack