Control over user:group ownership of files in docker volume?

Questions
1

Hi,

I have an odd situation. Kanboard is installed as docker container via docker-compose.yaml - host system is Debian. The application works fine, is connecting to my MariaDB database container and uses LDAP for auth.

Part of my docker compose file:

version: '2'
services:
  kanboard:
    image: kanboard/kanboard:v1.2.25
    container_name: kanboard
    hostname: kanboard
    ports:
      - "80:80"
      - "443:443"
    volumes:
      - kanboard:/var/www/app/data
      - kanboard_plugins:/var/www/app/plugins
      - kanboard_sslcerts:/etc/nginx/ssl
    labels:
      - ...
    environment:
      - PUID=1002
      - PGID=100
      - ...
    restart: unless-stopped
volumes:
  kanboard:
    external: true
  kanboard_plugins:
    external: true
  kanboard_sslcerts:
    external: true

What’s puzzling me is that on the container host side of things all files / folders in the volumes belong to user:group _app:systemd-journal. Looking from inside the container shell, user:group are nginx:nginx as expected.

The PUID / PGID environment variables in the docker compose file would result in the desired host system user:group combination but it isn’t applied. I’ve also tried UID / GID instead of PUID / PGID but the container doesn’t seem to recognize it.

Any idea what I’m mising? Thanks in advance.