Add scope input to set scopes for the authentication token

[crazy-max]

follow-up docker/buildx#3562

Adds scope input to create alternative Docker config on login that will be used by Buildx.

Example to scope auth token to specific repo:

  login:
    runs-on: ubuntu-latest
    steps:
      -
        name: Checkout
        uses: actions/checkout@v6
      -
        name: Login to Docker Hub
        uses: docker/login-action@v3
        with:
          username: ${{ vars.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}
          scope: docker/buildx-bin

Docker config files on GitHub Runner:

## /home/runner/.docker/buildx/config/registry-1.docker.io/docker/buildx-bin/config.json
{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "REDACTED"
    }
  }
}

## /home/runner/.docker/config.json
{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "REDACTED"
    }
  }
}

/home/runner/.docker/config.json is the default one available on the runner.

Other example with scope auth token to specific repo and only push:

  login:
    runs-on: ubuntu-latest
    steps:
      -
        name: Checkout
        uses: actions/checkout@v6
      -
        name: Login to Docker Hub
        uses: docker/login-action@v3
        with:
          username: ${{ vars.DOCKERHUB_USERNAME }}
          password: ${{ secrets.DOCKERHUB_TOKEN }}
          scope: docker/buildx-bin@push

Docker config files on GitHub Runner:

## /home/runner/.docker/buildx/config/registry-1.docker.io/docker/buildx-bin@push/config.json
{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "REDACTED"
    }
  }
}

## /home/runner/.docker/config.json
{
  "auths": {
    "https://index.docker.io/v1/": {
      "auth": "REDACTED"
    }
  }
}