[pan] fix (gitlab ci): set DEBIAN_FRONTEND noninteractive

[Dominique Dumont <ddumont src gnome org>]

commit b2e36e6da3e27c5d6af4f2a3fe3f8a8f3fd98e7c
Author: Dominique Dumont <dod debian org>
Date:   Wed Sep 1 17:36:42 2021 +0200

    fix (gitlab ci): set DEBIAN_FRONTEND noninteractive
    
    To avoid build job frozen on tzdata question.
    
    See 
https://askubuntu.com/questions/909277/avoiding-user-interaction-with-tzdata-when-installing-certbot-in-a-docker-contai

 .gitlab-ci.yml | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 9b8eb75e..2630f98c 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,3 +1,6 @@
+variables:
+  DEBIAN_FRONTEND: noninteractive
+
 build-ubuntu-gtk2:
   image: ubuntu:rolling
   before_script: