ci: add cache for docker image

.gitea/workflows/main.yaml

@@ -10,6 +10,15 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      # Cache Docker layers
+      - name: Cache Docker image layers
+        uses: http://fly6516.synology.me:4000/Gitea_Action/cache@v3
+        with:
+          path: /tmp/.buildkit-cache
+          key: ${{ runner.os }}-docker-${{ hashFiles('**/Dockerfile') }}
+          restore-keys: |
+            ${{ runner.os }}-docker-
+
       # Checkout the repository
       - name: Checkout repository
         uses: http://fly6516.synology.me:4000/Gitea_Action/checkout@v2