[gtk/asan-ci: 5/9] ci: Mention image layering in the README

[Matthias Clasen <matthiasc src gnome org>]

commit de8258171af8f27e3bc96fb2d8945bc19d447936
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jul 9 13:11:26 2020 -0400

    ci: Mention image layering in the README

 .gitlab-ci/README.md | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/.gitlab-ci/README.md b/.gitlab-ci/README.md
index 3958faf001..e7b063144f 100644
--- a/.gitlab-ci/README.md
+++ b/.gitlab-ci/README.md
@@ -35,4 +35,13 @@ branch, as well as their available versions.
  - [ ] Add the new job to `.gitlab-ci.yml` referencing the image
  - [ ] Open a merge request with your changes and let it run
 
+### Checklist for Adding a new dependency to a CI image
+
+Our images are layered, and the base (called fedora-base) contains
+all the rpm payload. Therefore, adding a new dependency is a 2-step
+process:
+
+ 1. [ ] Build and upload fedora-base:$version+1
+ 1. [ ] Build and upload fedora:$version+1 based on fedora-base:version+1
+
 [registry]: https://gitlab.gnome.org/GNOME/gtk/container_registry