[add] mysql performance improvement

docker-compose/docker-compose.yml

@@ -6,7 +6,7 @@ services:
     env_file: .env
     restart: always
     volumes:
-      - ./srv/invoiceninja/mysql:/var/lib/mysql
+      - data-volume:/var/lib/mysql
 
   app:
     image: invoiceninja/invoiceninja
@@ -47,3 +47,6 @@ services:
       done
       EOF'
 
+volumes:
+  data-volume:
+