Update third-party GitHub Actions to latest major versions

[Copilot]

docker/build-push-action@v1 is significantly out of date (current: v7). v2+ dropped inline auth inputs entirely, requiring companion actions for login and buildx setup.

Changes

• build-docker.yml: Replaces the two separate docker/build-push-action@v1 steps (which embedded credentials directly) with the modern pattern:
  • docker/setup-buildx-action@v4 — now required by v2+
  • docker/login-action@v4 — Docker Hub auth
  • docker/login-action@v4 — GitHub Packages auth
  • docker/build-push-action@v7 — single build+push to both registries using push: true + tags

Before:

- uses: docker/build-push-action@v1
  with:
    username: ${{ secrets.DOCKER_USERNAME }}
    password: ${{ secrets.DOCKER_PASSWORD }}
    repository: mltframework/melt
    tag_with_ref: true

After:

- uses: docker/setup-buildx-action@v4
- uses: docker/login-action@v4
  with:
    username: ${{ secrets.DOCKER_USERNAME }}
    password: ${{ secrets.DOCKER_PASSWORD }}
- uses: docker/build-push-action@v7
  with:
    push: true
    tags: mltframework/melt:${{ github.ref_name }}

All other third-party actions (msys2/setup-msys2@v2, lukka/run-vcpkg@v11, ilammy/msvc-dev-cmd@v1) are already at their latest major versions.

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