CI: build flatpak using the "official" Docker image

[skip ci]

.gitlab-ci.yml

@@ -132,7 +132,7 @@ pages:
 ##
 flatpak:
   stage: deploy
-  image: claudioandre/settings:fedora.flatpak
+  image: registry.gitlab.gnome.org/gnome/gnome-nightly-oci/nightly:master
   artifacts:
     name: package
     paths:

build-aux/flatpak/org.gnome.Settings.json

@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.Settings",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "3.28",
+    "runtime-version" : "master",
     "sdk" : "org.gnome.Sdk",
     "command" : "gnome-control-center",
     "rename-desktop-file" : "gnome-control-center.desktop",