CI: add attestation to ORAS artifacts

If consumers retrieve guest-components via OCI instead of building them
themselves, it makes sense to add attestations so we can assert on the
consumer side that they have been built from untampered upstream
sources on github runners.

Signed-off-by: Magnus Kulke <[email protected]>

.github/workflows/publish-artifacts.yml

@@ -14,6 +14,7 @@ jobs:
       contents: read
       packages: write
       id-token: write
+      attestations: write
     strategy:
       matrix:
         tee:
@@ -101,6 +102,7 @@ jobs:
       run: make ./target/${{ matrix.arch }}-unknown-linux-${{ matrix.libc}}/release/attestation-agent
 
     - name: Publish to ORAS
+      id: publish
       env:
         ARCH: ${{ matrix.arch == 'x86_64' && 'amd64' || matrix.arch }}
       run: |
@@ -118,12 +120,23 @@ jobs:
         docker manifest create "${image}:${tag}" --amend "${image}:${arch_tag}"
         docker manifest annotate --arch "$ARCH" --os linux "${image}:${tag}" "${image}:${arch_tag}"
         docker manifest push "${image}:${tag}"
+        # add image and digest to output for attestation
+        echo "image=${image}" >> "$GITHUB_OUTPUT"
+        digest="$(oras manifest fetch "${image}:${arch_tag}" | jq -r '.manifests[].digest')"
+        echo "digest=${digest}" >> "$GITHUB_OUTPUT"
+
+    - uses: actions/attest-build-provenance@v1
+      with:
+        subject-name: ${{ steps.publish.outputs.image }}
+        subject-digest: ${{ steps.publish.outputs.digest }}
+        push-to-registry: true
 
   publish-cdh-and-asr:
     permissions:
       contents: read
       packages: write
       id-token: write
+      attestations: write
     strategy:
       matrix:
         arch: