Enforce DCO sign-off in CI

Use a Github Action to check all commits in a pull request have a
Signed-off-by trailer.

Signed-off-by: Leonardo Forchini <leonardo.forchini@nutanix.com>

Author: lforchini

.github/workflows/pull_request.yaml

@@ -3,6 +3,10 @@ name: Pull Request
 on: pull_request
 
 jobs:
+  dco:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: KineticCafe/actions-dco@v3.2.0
   pre-push:
     strategy:
       matrix: