[gtk] ci: Update container to Fedora 28

[Emmanuele Bassi <ebassi src gnome org>]

commit d448be007ee2111794dcd9315a6441c6127c920c
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Apr 11 13:58:47 2018 +0100

    ci: Update container to Fedora 28
    
    We're going to need updated dependencies that we cannot build as
    subprojects, like wayland-protocols.

 .gitlab-ci/Dockerfile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci/Dockerfile b/.gitlab-ci/Dockerfile
index eaa0e6153d..84005c8324 100644
--- a/.gitlab-ci/Dockerfile
+++ b/.gitlab-ci/Dockerfile
@@ -1,4 +1,4 @@
-FROM fedora:27
+FROM fedora:28
 
 RUN dnf -y install \
     adwaita-icon-theme \