Update alpine Docker tag to v3.20

Signed-off-by: milkmaker <milkmaker@mailcow.de>

helper-scripts/docker-compose.override.yml.d/EXTERNAL_MYSQL_SOCKET/docker-compose.override.yml

@@ -26,6 +26,6 @@ services:
         - /var/run/mysqld/mysqld.sock:/var/run/mysqld/mysqld.sock
 
     mysql-mailcow:
-      image: alpine:3.19
+      image: alpine:3.20
       command: /bin/true
       restart: "no"