ci: Build custom image

The gnome-shell image we are using is well-suited for the jobs we
are running, but the lack of sassc means that we don't cover classic
mode.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/186>

.gitlab-ci.yml

@@ -1,10 +1,12 @@
 include:
+  - remote: 'https://gitlab.freedesktop.org/freedesktop/ci-templates/-/raw/bbe5232986c9b98eb1efe62484e07216f7d1a4df/templates/fedora.yml'
   - remote: "https://gitlab.freedesktop.org/freedesktop/ci-templates/-/raw/6f86b8bcb0cd5168c32779c4fea9a893c4a0c046/templates/ci-fairy.yml"
 
 image: registry.gitlab.gnome.org/gnome/gnome-shell/fedora/34:2021-08-12.0
 
 stages:
   - pre_review
+  - prepare
   - review
   - build
 
@@ -39,6 +41,18 @@ workflow:
     - if: '$CI_COMMIT_BRANCH =~ /^gnome-[0-9-]+$/'
     - when: 'manual'
 
+.gnome-shell-extensions.fedora:34:
+  variables:
+    FDO_DISTRIBUTION_VERSION: 34
+    FDO_DISTRIBUTION_TAG: '2021-08-31.0'
+    FDO_DISTRIBUTION_PACKAGES: >
+      meson git gettext sassc
+
+.prereview_req: &prereview_req
+  needs:
+    - check_commit_log
+    - check-merge-request
+
 check_commit_log:
   extends:
     - .fdo.ci-fairy
@@ -77,8 +91,16 @@ check-merge-request:
     reports:
       junit: check-merge-request-report.xml
 
+build-fedora-container:
+  extends:
+    - .fdo.container-build@fedora@x86_64
+    - .gnome-shell-extensions.fedora:34
+  stage: prepare
+  <<: *prereview_req
+
 js_check:
   stage: review
+  <<: *prereview_req
   script:
     - find extensions -name '*.js' -exec js78 -c '{}' ';' 2>&1 | tee $JS_LOG
     - (! grep -q . $JS_LOG)
@@ -89,6 +111,7 @@ js_check:
 
 eslint:
   stage: review
+  <<: *prereview_req
   script:
     - eslint -o $LINT_LOG -f junit --resolve-plugins-relative-to $(npm root -g) extensions
   artifacts:
@@ -99,7 +122,7 @@ eslint:
 
 build-bundles:
   stage: build
-  needs: ["check_commit_log"]
+  <<: *prereview_req
   script:
     - ./export-zips.sh
   artifacts: