ci: only run DCO check on merge requests

Signed-off-by: Dan Williams <dan@ioncontrol.co>

.gitlab-ci.yml

@@ -31,16 +31,15 @@ verify-dco:
   - .fdo.distribution-image@ubuntu
   - .common_variables
   only:
-    - main
     - merge_requests
-    - tags
-    - schedules
   script: |
-    export COMMITS=$(git log --no-merges --format="%H" ${CI_MERGE_REQUEST_DIFF_BASE_SHA}.. | xargs)
-    for c in "${COMMITS}"; do
-      echo "Checking commit ${c} for DCO Signed-off-by"
-      git log --format="%b" -1 ${c} | grep "Signed-off-by: .* <.*@.*>"
-    done
+    if [[ -n "${CI_MERGE_REQUEST_DIFF_BASE_SHA}" ]]; then
+      export COMMITS=$(git log --no-merges --format="%H" ${CI_MERGE_REQUEST_DIFF_BASE_SHA}.. | xargs)
+      for c in "${COMMITS}"; do
+        echo "Checking commit ${c} for DCO Signed-off-by"
+        git log --format="%b" -1 ${c} | grep "Signed-off-by: .* <.*@.*>"
+      done
+    fi
 
 build container:
   extends: