[gimp-web] gitlab-ci: add mirror consistency check.

[Jehan <jehanp src gnome org>]

commit 77ab90478aa3cfbf3c868f942f7e78f89f8b9f8a
Author: Jehan <jehan girinstud io>
Date:   Fri Sep 24 18:10:40 2021 +0200

    gitlab-ci: add mirror consistency check.
    
    We have 2 lists of mirrors:
    - tools/downloads/downloads.http.txt is the list as gathered
      programmatically from download.gimp.org (through update-mirrors.py
      script, though only people with SSH access can run it).
    - content/downloads/mirrors.json is manually curated and must have
      exactly the mirrors listed by the first script, with added
      organization names, URLs and location (which is why it needs to be
      manually curated). This is the file which is used to generate the
      Sponsors page.
    
    Since we want the Sponsors page to be always accurate, we should always
    verify the 2 files match. We don't want to forget a nice new mirror
    organization, nor do we want to keep listing one which stopped mirroring
    us (though maybe we could have a legacy list? Note sure how useful it is
    for mirrors).
    
    This is the first rule in our Gitlab CI. In the future, it would be nice
    to have build rules as well (and probably more checks when relevant).

 .gitlab-ci.yml | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
new file mode 100644
index 00000000..b931003a
--- /dev/null
+++ b/.gitlab-ci.yml
@@ -0,0 +1,16 @@
+image: debian:testing
+
+stages:
+  - check
+
+check-mirrors:
+  stage: check
+  before_script:
+    - apt-get update
+    - apt-get install -y --no-install-recommends
+        python3
+  script:
+    # Check that the mirror lists are consistent so that the sponsors
+    # page always display exactly the right list of mirrors (no more no
+    # less compared to the ones configured on download mirror rotation).
+    - tools/downloads/cmp-mirrors.py