[geary/mjog/gnome-3-36-ci] .gitlab-ci.yml: Fix stable Flatpak SDK not being found

[Michael Gratton <mjog src gnome org>]

commit 62ce6646e96e504c4cbc3c0d137a55f9b43a051e
Author: Michael Gratton <mike vee net>
Date:   Mon Oct 7 13:43:29 2019 +1100

    .gitlab-ci.yml: Fix stable Flatpak SDK not being found

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 9edfe730..c7584bd3 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -84,7 +84,7 @@ ubuntu:
     expire_in: 2 days
 
 flatpak:
-  image: 'registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:master'
+  image: 'registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:3.36'
   variables:
     APP_ID: org.gnome.Geary.Devel
     FLATPAK_MODULE: geary