[gnome-control-center] ci: Build Docker images when Dockerfile changes

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 1d4d9953230d727b4d6448ac4422e9d8b518ea7f
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Sun Nov 18 20:53:59 2018 -0200

    ci: Build Docker images when Dockerfile changes

 .gitlab-ci.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 89254b9bd..703338e1a 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -317,8 +317,8 @@ ppc64le:
   services:
     - docker:dind
   only:
-    variables:
-      - $CRON_TASK == "BUILD_CI_IMAGES"
+    changes:
+      - '.gitlab-ci/Dockerfile'
 
   script:
     # Skip the build (if requested)