ci-dockerfiles-sanity-check: Trigger on "check-staging" gerrit comment

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I3bff80b219760d8c5d12167152d584f3d488fe03
diff --git a/ci-dockerfiles-sanity-check.yaml b/ci-dockerfiles-sanity-check.yaml
index e51bd53..b29ec14 100644
--- a/ci-dockerfiles-sanity-check.yaml
+++ b/ci-dockerfiles-sanity-check.yaml
@@ -37,14 +37,15 @@
         - gerrit:
             server-name: 'review.trustedfirmware.org'
             trigger-on:
-                - patchset-created-event
+                - comment-added-contains-event:
+                    comment-contains-value: check-staging
             projects:
                 - project-compare-type: 'PLAIN'
                   project-pattern: 'ci/dockerfiles'
                   branches:
                     - branch-compare-type: 'PLAIN'
                       branch-pattern: 'master'
-            silent-start: true
+            silent-start: false
     axes:
         - axis:
             type: slave