Fix Docker CI: switch from Docker Hub to GitHub Container Registry

[Copilot]

The docker CI job was failing because DOCKERHUB_TOKEN has read-only scope, causing push to registry-1.docker.io to fail with insufficient_scope: authorization failed.

Changes

• Both push_master.yml and release.yml: Replace Docker Hub login (DOCKERHUB_USERNAME/DOCKERHUB_TOKEN) with GitHub Container Registry login (ghcr.io via GITHUB_TOKEN)
• Image tags: Updated from dappnode/mev-sp-oracle:* → ghcr.io/dappnode/mev-sp-oracle:*
• Permissions: Added packages: write to both jobs (required for ghcr.io push)

permissions:
  contents: read
  packages: write
# ...
- name: Login to GitHub Container Registry
  uses: docker/login-action@v3
  with:
    registry: ghcr.io
    username: ${{ github.actor }}
    password: ${{ secrets.GITHUB_TOKEN }}

GITHUB_TOKEN is automatically scoped with push access to the repository's packages, eliminating the dependency on externally-managed Docker Hub credentials.