Add dummy job to manually trigger a pipeline

.gitlab/ci_templates/dummy_job.yml

+dummy-job:
+  script:
+  - echo "This is a dummy job which does nothing else but triggers a pipeline"
+  - echo "nonetheless, useful for situations where CI rules would not trigger"
+  - echo "a pipeline automatically, i.e. changes to renovate.json"
+  - echo "(see https://open.greenhost.net/stackspin/wordpress-helm/-/merge_requests/127#note_38728)"
+  when: manual