Update test-docker.yml

Update actions/checkout from v2 to v3

.github/workflows/test-docker.yml

@@ -7,7 +7,7 @@ jobs:
   docker:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
       with:
         submodules: 'recursive'
       name: Checkout