gitlab-ci: Introduce sign-off check

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

.gitlab-ci.yml

@@ -0,0 +1,18 @@
+variables:
+  GITLAB_CI_IMAGE_ALPINE: 'alpine:3.16.0'
+stages:
+  - static
+
+# 'static' stage
+sign_off:
+  stage: static
+  needs: []
+  image: ${GITLAB_CI_IMAGE_ALPINE}
+  before_script:
+    - apk add --no-cache git
+  script: >-
+    git log
+    --grep "^Signed-off-by: .\+<.\+\(@\| at \).\+\(\.\| dot \).\+>$"
+    --invert-grep
+    --format="Detected commit '%h' with missing or bad sign-off! Please read 'CONTRIBUTING.md'."
+    --exit-code