[gnome-menus] ci: switch to debian-slim image since it's 10x smaller

[Jeremy Bicha <jbicha src gnome org>]

commit 6a55ae5ed0bc3000cbab4d4b61b2a4ba10d98bbd
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Fri Jan 25 23:04:19 2019 -0500

    ci: switch to debian-slim image since it's 10x smaller
    
    than gnome-runtime base
    
    Ubuntu has an even smaller image but Debian is good enough here for now

 .gitlab-ci.yml | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index c06ad0d..ae276a1 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,12 +1,13 @@
 validate-menu:
-  image: 'registry.gitlab.gnome.org/gnome/gnome-runtime-images/base:latest'
+  image: debian:unstable-slim
   # libxml2 doesn't support https yet but the spec is only available via https
   # https://bugzilla.gnome.org/791220
   # https://gitlab.freedesktop.org/freedesktop/freedesktop/issues/114
   # We shouldn't add the dtd to our git repo because it doesn't have
   # a clearly distributable license
   before_script:
-    - dnf install -y wget
+    - apt-get update
+    - apt-get -y install libxml2-utils wget
   script:
     - wget https://standards.freedesktop.org/menu-spec/menu-latest.dtd
     - xmllint --noblanks --noout --path menu-latest.dtd layout/gnome-applications.menu