ci: reinstate gha cache

.github/workflows/docker.yml

@@ -85,6 +85,14 @@ jobs:
           username: ${{ env.REGISTRY_USER }}
           password: ${{ env.REGISTRY_PASS }}
 
+      - name: Set up Docker Buildx cache
+        uses: actions/cache@v4
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
+
       - name: Build and push
         uses: docker/build-push-action@v6
         with:
@@ -93,3 +101,5 @@ jobs:
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max