Migrate CI image publishing from Docker Hub to GitHub Container Registry

[Copilot]

The CI pipeline was pushing Docker images to Docker Hub under a hardcoded personal account (wurstbrot) using an external HUB_TOKEN secret — not appropriate for an enterprise setup.

Changes

• Registry: Docker Hub → ghcr.io (GitHub Container Registry)
• Auth: secrets.HUB_TOKEN + hardcoded username → secrets.GITHUB_TOKEN + github.actor (no manual secret management required)
• Image tags: wurstbrot/dsomm* → ghcr.io/${{ github.repository_owner }}/dsomm*
• Permissions: Added packages: write to the job to allow GHCR pushes

- name: Log in to GitHub Container Registry
  uses: docker/login-action@v2
  with:
    registry: ghcr.io
    username: ${{ github.actor }}
    password: ${{ secrets.GITHUB_TOKEN }}

Images are now owned by the repository's organization/owner, authenticated via the automatically-provisioned GITHUB_TOKEN, with access control managed through GitHub's permission model.

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