ci: Temporarily neutralize image build job

It's failing CI too often, and too consistently. This should be
reverted once CI failures are gone.

Part-of:
<https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3569>

.gitlab-ci.yml

@@ -24,7 +24,7 @@ build-sysext:
 
 stages:
   - pre_review
-  - prep
+# - prep
   - review
   - build
   - test
@@ -140,11 +140,11 @@ check-merge-request:
     reports:
       junit: check-merge-request-report.xml
 
-build-fedora-container:
+#build-fedora-container:
-  extends:
+#  extends:
-    - .fdo.container-build@fedora@x86_64
+#    - .fdo.container-build@fedora@x86_64
-    - .gnome-shell.fedora
+#    - .gnome-shell.fedora
-  stage: prep
+#  stage: prep
 
 js_check:
   extends: