ci(docker): add buildx cache for PR smoke on blacksmith (#695)

.github/workflows/docker.yml

@@ -28,6 +28,7 @@ jobs:
         runs-on: blacksmith-2vcpu-ubuntu-2404
         timeout-minutes: 25
         permissions:
+            actions: write
             contents: read
         steps:
             - name: Checkout repository
@@ -50,9 +51,15 @@ jobs:
                   context: .
                   push: false
                   load: true
+                  provenance: false
+                  sbom: false
                   tags: zeroclaw-pr-smoke:latest
                   labels: ${{ steps.meta.outputs.labels }}
                   platforms: linux/amd64
+                  cache-from: |
+                      type=gha,scope=zeroclaw-docker-smoke
+                  cache-to: |
+                      type=gha,mode=max,scope=zeroclaw-docker-smoke
 
             - name: Verify image
               run: docker run --rm zeroclaw-pr-smoke:latest --version