Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -42,6 +42,8 @@ Build_Windows:
 Build_Linux:
     only: ['web', 'schedules']
     extends: .Build_Linux_
+    # override image from base script as there is no official 4.26 image
+    image: registry.git-ce.rwth-aachen.de/vr-vis/vr-group/unreal-development/unreal-docker-ci-images/4-26_linux
     tags:
         - linux
         - unreal