ci: detect [skip ci] in PR descriptions and prevent breakage

When Jenkins jobs are started by a comment, and if the PR contains
`[skip ci]` in the description, Jenkins does not run the requested job,
nor set a status for the job in the PR.

This causes Mergify to add the `ok-to-test` label again, instructing a
GitHub Action to add comments to start jobs in Jenkins. Once all
comments have been posted, the `ok-to-test` label is removed. Mergify
then notices that the jobs were not run, and adds the `ok-to-test` label
again.... Endlessly looping of adding the label, commenting and removing
the label as a result.

By reporting the brokenness of the PR description and marking the PR as
Draft, the looping is prevented.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

.mergify.yml

@@ -464,3 +464,17 @@ pull_request_rules:
           - Repo activity
           - ci/skip/e2e
           - ci/skip/multi-arch-build
+
+  # A [skip ci] in the PR description prevents Jenkins jobs from running, mark
+  # the PR as Draft so that CI jobs do not automatically run anymore.
+  - name: detect [skip ci] in the PR description
+    conditions:
+      - "body-raw~=[skip ci]"
+    actions:
+      edit:
+        draft: true
+      comment:
+        # yamllint disable-line rule:truthy
+        message: "The PR description contains the unsupported `[skip ci]`
+        command, please update the description and mark the PR ready for review
+        again."