building a singularity image from docker hub postgres image

I am trying to build a singularity image from postgres docker image.
I am issuing the command below.

$ singularity build /local/data/some/postgres.16.1.sif
docker://postgres/postgres:16.1

INFO:    Starting build...
INFO:    Fetching OCI image...
FATAL:   While performing build: conveyor failed to get: GET
https://index.docker.io/v2/postgres/postgres/manifests/16.1: UNAUTHORIZED:
authentication required; [map[Action:pull Class: Name:postgres/postgres
Type:repository]]


What is the url I should use?

-Allan.