new image workflow to upload to ghcr

[afbase]

Summary

1. Modified rsky-pds/Dockerfile to use multi-stage builds like the jetstream subscriber, while preserving the
   environment variables in the CMD
2. Created .github/workflows/ghcr.yml that will publish the Docker images to ghcr.io for the blacksky-algorithms
   organization

The workflow will trigger when changes are pushed to main branch that affect any of the three services, or
manually via workflow_dispatch. It builds and pushes each Docker image with appropriate tags based on the git
reference.

Related Issues

I'm trying to build an rsky-pds instance in k8s.

Changes

• Feature implementation
• Bug fix
• Documentation update
• Other (please specify):

Checklist

• I have tested the changes (including writing unit tests).
• I confirm that my implementation aligns with the canonical Typescript implementation and/or atproto spec
• I have updated relevant documentation.
• I have formatted my code correctly
• I have provided examples for how this code works or will be used