build: Attempt to fix cache warnings

Author: Alan Christie

.github/workflows/build.yaml

@@ -52,9 +52,18 @@ jobs:
       uses: docker/setup-qemu-action@v3
     - name: Set up Docker Buildx
       uses: docker/setup-buildx-action@v3
+    - name: Cache Docker layers
+      uses: actions/cache@v4
+      with:
+        path: /tmp/.buildx-cache
+        key: ${{ runner.os }}-buildx-${{ github.sha }}
+        restore-keys: |
+          ${{ runner.os }}-buildx-
     - name: Build
       uses: docker/build-push-action@v6
       with:
         context: .
         platforms: linux/amd64,linux/arm64
         tags: ${{ github.repository_owner }}/squonk2-fastapi-ws-event-stream:${{ env.GITHUB_REF_SLUG }}
+        cache-from: type=local,src=/tmp/.buildx-cache
+        cache-to: type=local,dest=/tmp/.buildx-cache