Nextcloud not using database

I have set up my Nextcloud server using docker-compose following the official examples and some guides I found on YouTube. Everything starts up correctly and works. In the settings of Nextcloud I nonetheless get a warning that says Nextcloud is using SQLite. Why isn’t Nextcloud using my mySQL database?

My docker-compose.yml file looks like this:

  nextcloud:
    container_name: nextcloud
    image: nextcloud:latest
    restart: always
    depends_on:
      - nextcloud_db
    ports:
      - 8088:80
    volumes:
      - C:\docker\nextcloud\html:/var/www/html:rw
      - Z:\Nextcloud\data:/var/www/html/data:rw
    environment:
      - MYSQL_HOST=nextcloud_db
      - MYSQL_DATABASE=nextcloud_db
      - MYSQL_USER=someUser
      - MYSQL_PASSWORD=somePassword

  nextcloud_db:
    container_name: nextcloud_db
    image: mariadb:latest
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - C:\docker\nextcloud_db\mysql:/var/lib/mysql
    environment:
      - MYSQL_DATABASE=nextcloud_db
      - MYSQL_ROOT_PASSWORD=someRootPassword
      - MYSQL_USER=someUser
      - MYSQL_PASSWORD=somePassword