That’s normally the wrong permission. You can try to change the permissions again on the host or you can add to the db-nextcloud service:
user: "999:999" # Run as non-root user
You can edit the docker compose with:
You could also manually create the directory and don’t let docker do it before installing the nextcloud:
sudo mkdir -p /DATA/AppData/big-bear-nextcloud/pgdata
sudo chown -R 999:999 /DATA/AppData/big-bear-nextcloud/pgdata
sudo chmod -R 700 /DATA/AppData/big-bear-nextcloud/pgdata