[tracker] .gitlab-ci.yml: Add comment about CI image

[Sam Thursfield <sthursfield src gnome org>]

commit 1e62f24e62bb1ed9c4a26333c93f3ae6732e3d16
Author: Sam Thursfield <sam afuera me uk>
Date:   Sun Sep 22 19:38:00 2019 +0200

    .gitlab-ci.yml: Add comment about CI image

 .gitlab-ci.yml | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 4754cee55..fffda6118 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,3 +1,6 @@
+# The container images used in this pipeline are built from this
+# GitLab project: https://gitlab.gnome.org/GNOME/tracker-oci-images
+
 variables:
   # These can be used to see verbose log output from the functional-tests.
   # See HACKING.md for more information.