chore(templates): publish with-postgres image to ghcr

[yangchuansheng]

What

• add a dedicated GitHub Actions workflow to publish the templates/with-postgres starter to GHCR
• add a dedicated multi-stage Docker build plus migration-aware entrypoint for that published image
• document the official image name and tag strategy in the template README

Why

Today users who want to deploy the official with-postgres starter as a container need to maintain their own image build automation around this repository.

Publishing an official image from the upstream repository makes the starter easier to deploy on container platforms, gives users a stable image source, and keeps the image lifecycle aligned with Payload releases.

Notes

• release builds publish latest, <payload-version>, and <payload-version>-<short-sha>
• main branch builds publish main and sha-<short-sha>
• workflow_dispatch supports custom refs and dry runs with push=false
• the image is intentionally named ghcr.io/payloadcms/payload-with-postgres so it is clearly scoped to the starter being published here