ci: dco job should run only on PRs (#396)

See also message in b919279eef

Signed-off-by: Andrei Aaron <aaaron@luxoft.com>

.github/workflows/dco.yml

@@ -2,16 +2,18 @@
 name: DCO
 on:
   pull_request:
-  push:
     branches:
       - main
+
+permissions: read-all
+
 jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - name: Set up Python 3.x
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v4
       with:
         python-version: '3.x'
     - name: Check DCO