Fix Docker Hub build: use postgres:19beta1 base image

[jrgemignani]

PostgreSQL 19 is still in beta, so the official Docker Hub postgres image has no 19 tag yet -- only 19beta1. The Docker Hub autobuild failed with:

postgres:19: not found
failed to resolve source metadata for docker.io/library/postgres:19

Point both docker/Dockerfile (build and final stages) and docker/Dockerfile.dev at postgres:19beta1 until the PG19 GA image is published.

Co-authored-by: Copilot copilot@github.com

modified: docker/Dockerfile
modified: docker/Dockerfile.dev

[Copilot]

Pull request overview

This PR updates the Docker base images to use the official PostgreSQL 19 beta tag available on Docker Hub, fixing Docker Hub autobuild failures caused by the non-existent postgres:19 tag during the PG19 beta period.

Changes:

• Switch docker/Dockerfile build and final stages from postgres:19 to postgres:19beta1.
• Switch docker/Dockerfile.dev from postgres:19 to postgres:19beta1.
• Add inline comments documenting why the beta tag is used and when to revert to postgres:19.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docker/Dockerfile	Updates both build and final stages to postgres:19beta1 and documents the temporary beta-tag rationale.
docker/Dockerfile.dev	Updates the dev image base to postgres:19beta1 and documents the temporary beta-tag rationale.

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