attest provenance for keycloak image

see https://docs.github.com/en/actions/security-for-github-actions/using-artifact-attestations/using-artifact-attestations-to-establish-provenance-for-builds#generating-build-provenance-for-container-images

Author: overheadhunter

.github/workflows/keycloak.yml

@@ -18,6 +18,11 @@ jobs:
   build-image:
     name: Build Custom Keycloak Image
     runs-on: ubuntu-latest
+    permissions:
+      id-token: write
+      contents: read
+      attestations: write
+      packages: write
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
@@ -42,10 +47,17 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
       - name: Build and Push Container Image
+        id: push
         uses: docker/build-push-action@v6
         with:
           context: keycloak
           platforms: linux/amd64,linux/arm64/v8
           push: true
           tags: |
-            ghcr.io/cryptomator/keycloak:${{ github.event.inputs.tag }}
+            ghcr.io/cryptomator/keycloak:${{ github.event.inputs.tag }}
+      - name: Generate artifact attestation
+        uses: actions/attest-build-provenance@v2
+        with:
+          subject-name: ghcr.io/cryptomator/keycloak
+          subject-digest: ${{ steps.push.outputs.digest }}
+          push-to-registry: true