image-build-infrastructure: Mark as obsolete

This document describes how we built images on Jenkins, but we have now
largely moved away from it in favor of GitLab CI/CD.

The README in the infrastructure/apertis-image-recipes> project
describes the current setup closer to the actual code.

Keep the document around since v2019 and v2020 are still officially on
Jenkins and old release notes still point here.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

content/architecture/long-term-reproducibility.md

@@ -100,9 +100,9 @@ they get triggered.
 In some cases, losing access to the build environment effectively means that
 a project can't be maintained anymore, as no new build can be made.
 
-To be able to avoid these issues as much as possible, Apertis makes heavy use
-of [isolated containers based on Docker
-images](image-build-infrastructure.md#docker-images-for-the-build-environment).
+To be able to avoid these issues as much as possible, Apertis makes heavy
+use of
+[isolated containers based on Docker images](https://gitlab.apertis.org/infrastructure/apertis-image-recipes/#building-in-docker)
 
 All the Apertis build pipelines run in containers with minimal access to
 external resources to keep the impact of the environment as low as possible.
@@ -145,7 +145,7 @@ and the main build process.
 ## Build recipes
 
 The process to the reference images is described by textual, YAML-based
-[Debos recipes]( {{< ref "image-build-infrastructure.md#image-building-process" >}} ) stored in
+[Debos recipes](https://gitlab.apertis.org/infrastructure/apertis-image-recipes/)
 git repository, with a different branch for each release channel.
 
 The textual, YAML-based GitLab-CI pipeline definitions then control how the

content/designs/image-build-infrastructure.md

 +++
-title = "Image building infrastructure"
+title = "Image building infrastructure (obsolete)"
 short-description = "Overview of the image build infrastructure for Apertis"
 weight = 100
 aliases = [
@@ -12,7 +12,13 @@ outputs = [ "html", "pdf-in",]
 date = "2018-01-10"
 +++
 
-# Apertis image build infrastructure overview
+{{% notice warning %}}
+This document provides and overview of the image build pipeline prior to the
+migration to GitLab CI/CD that has been completed during the v2021
+development cycle. Refer to the documentation in the
+[infrastructure/apertis-image-recipes](https://gitlab.apertis.org/infrastructure/apertis-image-recipes/)
+project for information about the current pipeline.
+{{% /notice %}}
 
 ## Introduction