Nextcloud is pretty hard to deploy, I regret having ever said it was easy. The good thing is there are some deployment examples on nextcloud/docker github conveniently placed under .examples/ (where of course they would be hidden).

Here's the example compose file I got to run on my machine docker-compose/insecure/mariadb/apache

The directory structure looks like this

[insecure/mariadb/apache]$ ls
db.env  docker-compose.yml

Here's the docker-compse file

# docker-compose.yml
version: '3'

services:
  db:
    image: mariadb:10.5
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    restart: always
    volumes:
      - db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=hackme
      - MARIADB_AUTO_UPGRADE=1
      - MARIADB_DISABLE_UPGRADE_BACKUP=1
    env_file:
      - db.env

  redis:
    image: redis:alpine
    restart: always

  app:
    image: nextcloud:apache
    restart: always
    ports:
      - 127.0.0.1:8080:80
    volumes:
      - nextcloud:/var/www/html
    environment:
      - MYSQL_HOST=db
      - REDIS_HOST=redis
    env_file:
      - db.env
    depends_on:
      - db
      - redis

  cron:
    image: nextcloud:apache
    restart: always
    volumes:
      - nextcloud:/var/www/html
    entrypoint: /cron.sh
    depends_on:
      - db
      - redis

volumes:
  db:
  nextcloud:

and here's the db.env file

MYSQL_PASSWORD=hackme
MYSQL_DATABASE=nextcloud
MYSQL_USER=nextcloud

The instructions from the example README.md are as follows

1. open `docker-compose.yml`
   1. insert your nextcloud domain behind `VIRTUAL<sub>HOST</sub>=`and `LETSENCRYPT<sub>HOST</sub>=`
   2. enter a valid email behind `LETSENCRYPT<sub>EMAIL</sub>=`
   3. if you use mariadb or mysql choose a root password for the database behind `MYSQL<sub>ROOTPASSWORD</sub>=`
2. choose a password for the database user nextcloud in `db.env` behind `MYSQL<sub>PASSWORD</sub>=` (for mariadb/mysql) or `POSTGRES<sub>PASSWORD</sub>=` (for postgres)
3. run `docker-compose build –pull` to pull the most recent base images and build the custom dockerfiles
4. start nextcloud with `docker-compose up -d`

I already added in a password in the above examples (hackme (not recommended btw))

$ docker-compose build --pull
$ docker compose up

I left off the -d for now so I could see all the amazing things docker's doing for me.

I tested it by creating a user, uploading a file and shutting down the server and then checking to see if everything worked fine after I restarted it. Everything appears to be fine.