Document how to run with specified user

[bmaehr]

According to #1812 it is possible to run the docker images with a specivied user. I was not able to make it working.

I use docker compose tu run the images and added

services:
  nextcloud-fpm:
    image: nextcloud:fpm-alpine
    user: 33:33

where 33 is the id of www-data on the host system.

But I still get the mesage that config.php is read-only.

If i do a docker exec -it cloud-nextcloud-fpm-1 sh Im abel to write config.php, but not if I do docker compose exec --user www-data nextcloud-fpm sh. This gives me

/var/www/html $ whoami
whoami: unknown uid 82

I'm confused.

Is it caused by

    volumes:
      - /etc/localtime:/etc/localtime:ro
      - /etc/passwd:/etc/passwd:ro
      - /etc/group:/etc/group:ro