Upgrade using Docker errors out with General error: 1018 Can't read dir of './nextcloud/' (errno: 13 "Permission denied")

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Recently pulled the latest docker image and am getting this error trying to update from the homescreen.

Preparing update

Set log level to debug

Turned on maintenance mode

Repair step: Repair MySQL collation

Repair info: All tables already have the correct collation -> nothing to do

Repair step: Repair SQLite autoincrement

Repair step: Copy data from accounts table when migrating from ownCloud

OC\DB\Exceptions\DbalException: An exception occurred while executing a query: An exception occurred while executing a query: SQLSTATE[HY000]: General error: 1018 Can't read dir of './nextcloud/' (errno: 13 "Permission denied")

I tried changing permissions, but no luck:

chown -R www-data:www-data /var/www/html/
chmod -R +rw /var/www/html/

I’m not sure where the ./nextcloud dir is supposed to be and what permissions it needs.

docker-compose.yml

version: '2'

volumes:
  nextcloud:
  db:

services:
  db:
    image: mariadb
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - /mnt/md0/nextclouddb:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=<pwd>
      - MYSQL_PASSWORD=<pwd>
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud

  app:
    build: .
    restart: always
    ports:
      - 8080:80
    links:
      - db
    volumes:
      - nextcloud:/var/www/html
    environment:
      - MYSQL_PASSWORD=<pwd>
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_HOST=db
      - NEXTCLOUD_OVERWRITEPROTOCOL=https

Dockerfile

FROM nextcloud

RUN apt update
RUN apt install ffmpeg imagemagick ghostscript -y

I have a feeling it’s because the external folder mnt/md0/nextclouddb is owned by root.