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>