Docker install on shared Postgres DB with unpriviledged user

I can鈥檛 find a way to script a docker-compose file which would use an existing Postgres instance.

The way I understand the documentation docs/nextcloud at master 路 docker-library/docs 路 GitHub is that I necessarily have to provide POSTGRES_DB, POSTGRES_USER, POSTGRES_PASSWORD the same as the postgres container instance, so that the default Postgres db and default Posgres superadmin user & pwd becomes the nextcloud user and password.

Am I understanding the doc wrong ?
Or do I have a way to specify necessary Superadmin user & pwd so that Nextcloud can create it own user & pwd?