docs : Add code checkout in docker CI

Author: niuoruo

.github/workflows/docker-CI.yaml

@@ -13,16 +13,16 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+    - name: Checkout code
+      uses: actions/checkout@v2
+
     - name: Log in to GitHub Container Registry
       uses: docker/login-action@v2
       with:
         registry: ghcr.io
         username: ${{ github.actor }}
         password: ${{ secrets.GITHUB_TOKEN }}
 
-    - name: Start SSH Service
-      uses: mxschmitt/action-tmate@v2
-
     - name: Build Docker image
       run: |
         docker build -t ghcr.io/irobot-algorithm/sentry_navigation:latest .