[gnome-applets/wip/muktupavels/icons] ci: add DEBIAN_FRONTEND environment variable

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 7577c967466f1f2ca83c0b748104db352132dbd2
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Tue Mar 24 15:54:13 2020 +0200

    ci: add DEBIAN_FRONTEND environment variable

 .gitlab-ci.yml | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6a7e4e865..1d6fb2ffb 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,6 +2,9 @@ stages:
   - build
   - analysis
 
+variables:
+  DEBIAN_FRONTEND: noninteractive
+
 build-ubuntu:
   image: ubuntu:devel
   stage: build