Mapping files folder outside the container (#6598)

In order to prevent the loss of files uploaded by users when upgrading Misskey deployed with Docker.
**But** it might be necessary to create the folder before `docker-compose up -d` (Not fully tested)

docker-compose.yml

@@ -13,6 +13,8 @@ services:
     networks:
       - internal_network
       - external_network
+    volumes:
+      - ./files:/misskey/files
 
   redis:
     restart: always