fix path confusion: move mysql data dir out of app data

docker-compose.yml

@@ -21,7 +21,7 @@ services:
       # Uncomment this if IPv6 is not enabled on your host
       # DISABLE_IPV6: 'true'
     volumes:
-      - ./${DATA_DIR}/data:/data
+      - ./${DATA_DIR}/app:/data
       - ./${DATA_DIR}/letsencrypt:/etc/letsencrypt
     depends_on:
       - db
@@ -35,4 +35,4 @@ services:
       MYSQL_USER: ${DB_USER}
       MYSQL_PASSWORD: ${DB_PASSWD}
     volumes:
-      - ./${DATA_DIR}/data/mysql:/var/lib/mysql
+      - ./${DATA_DIR}/mysql:/var/lib/mysql