ci: Check repo sanity

Check that the container registry in user repositories is
enabled, and issue a more descriptive error around it. Lifted
from Tracker.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3681>

.gitlab-ci.yml

@@ -237,6 +237,29 @@ workflow:
     - if: $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
       when: on_success
 
+repo-sanity:
+  extends:
+    - .fdo.ci-fairy
+  stage: review
+  variables:
+    GIT_DEPTH: "1"
+  script:
+    - >
+      if [[ -z "$CI_REGISTRY_IMAGE" ]] ;
+      then
+        .gitlab-ci/simple-junit-report.sh check-junit-report.xml \
+          repo-sanity "The container registry should be enabled in the project general settings panel at $CI_PROJECT_URL/edit" ;
+        exit 1 ;
+      fi
+  artifacts:
+    expire_in: 1 week
+    paths:
+      - check-junit-report.xml
+    reports:
+      junit: check-junit-report.xml
+  rules:
+    - !reference [.only-merge-requests, rules]
+
 check-commit-log:
   extends:
     - .fdo.ci-fairy

.gitlab-ci/simple-junit-report.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+OUTFILE=$1
+NAME=$2
+MESSAGE=$3
+
+cat >$OUTFILE <<EOF
+<?xml version='1.0' encoding='utf-8'?>
+<testsuites tests="1" errors="0" failures="1">
+  <testsuite name="repo-sanity" tests="1" errors="0" failures="1">
+    <testcase name="$NAME" classname="repo-sanity">
+      <failure message="$MESSAGE"/>
+    </testcase>
+  </testsuite>
+</testsuites>
+EOF
+
+# Also echo the message in stdout for good measure
+echo $MESSAGE