Forbiden to upload on external storage

hi,
i’m currenly install nextcloud instace on docker
and i add external volume by bind mounting on /media/externals

i try to use same volume to another container and can write file just fine, but after i try on nextcloud it just popup message forbiden

plz help me

here my docker compose
i just copy paste official docker container image and add one volume

version: '2'

volumes:
  nextcloud:
  db:
  externals_data:
    external: true
    
services:
  db:
    image: mariadb:10.5
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=dummypassword
      - MYSQL_PASSWORD=dummypassword
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud

  app:
    image: nextcloud
    restart: always
    ports:
      - 8080:80
    links:
      - db
    volumes:
      - nextcloud:/var/www/html
      - externals_data:/media/externals
    environment:
      - MYSQL_PASSWORD=dummypassword
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_HOST=db