Use reglient:
src="ghcr.io/iximiuz/labs/nginx:alpine"
dst="us-central1-docker.pkg.dev/dml-sandbox-firebass-21/demorepo/nginx:alpine"
curl -LO https://github.com/regclient/regclient/releases/download/v0.8.0/regctl-linux-amd64
chmod +x regctl-linux-amd64