Revert "ci: Temporarily neutralize image build job"

This reverts commit 942f7e87be41c5507c9127d69922f7b432ea729a.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3570>

.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:
-#  extends:
-#    - .fdo.container-build@fedora@x86_64
-#    - .gnome-shell.fedora
-#  stage: prep
+build-fedora-container:
+  extends:
+    - .fdo.container-build@fedora@x86_64
+    - .gnome-shell.fedora
+  stage: prep
 
 js_check:
   extends: