Add build + publish + SLSA workflow for attestor

Signed-off-by: Raghav Kaul <[email protected]>

.github/workflows/attestor.yaml

@@ -0,0 +1,67 @@
+on:
+  push:
+    tags:
+      - "*" # triggers only if push new tag version, like `0.8.4`
+
+env:
+  IMAGE_REGISTRY: "us-east4-docker.pkg.dev/openssf/"
+  IMAGE_NAME: "scorecard-attestor"
+
+jobs:
+  build:
+    permissions:
+      contents: read
+    outputs:
+      image: ${{ steps.image.outputs.image }}
+      digest: ${{ steps.build.outputs.digest }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@2541b1294d2704b0964813337f33b291d3f8596b # v2.3.4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@dc7b9719a96d48369863986a06765841d7ea23f6 # v2.0.0
+
+      - name: Authenticate Docker
+        uses: docker/login-action@49ed152c8eca782a232dede0303416e8f356c37b # v2.0.0
+        with:
+          registry: ${{ env.IMAGE_REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }} # FIXME - need GCB token stuff
+
+      - name: Extract metadata (tags, labels) for Docker
+        id: meta
+        uses: docker/metadata-action@69f6fc9d46f2f8bf0d5491e4aabe0bb8c6a4678a # v4.0.1
+        with:
+          images: ${{ env.IMAGE_REGISTRY }}/${{ env.IMAGE_NAME }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@e551b19e49efd4e98792db7592c17c09b89db8d8 # v3.0.0
+        id: build
+        with:
+          context: .
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          push: true
+
+      - name: Output image
+        id: image
+        run: |
+          # NOTE: Set the image as an output because the `env` context is not
+          # available to the inputs of a reusable workflow call.
+          image_name="${IMAGE_REGISTRY}/${IMAGE_NAME}"
+          echo "image=$image_name" >> "$GITHUB_OUTPUT"
+  provenance:
+    needs: [build]
+    permissions:
+      actions: read # for detecting the Github Actions environment.
+      id-token: write # for creating OIDC tokens for signing.
+      packages: write # for uploading attestations.
+    if: startsWith(github.ref, 'refs/tags/')
+    uses: slsa-framework/slsa-github-generator/.github/workflows/[email protected]
+    with:
+      image: ${{ needs.build.outputs.image }}
+      digest: ${{ needs.build.outputs.digest }}
+      registry-username: ${{ github.actor }}
+    secrets:
+      registry-password: ${{ secrets.GITHUB_TOKEN }}