Update docker.yml

Author: liangliangyy

.github/workflows/docker.yml

@@ -18,6 +18,13 @@ jobs:
         uses: docker/setup-qemu-action@v1
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
+      - name: Cache Docker layers
+        uses: actions/cache@v2
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
       - name: Login to DockerHub
         uses: docker/login-action@v1
         with: