[gnome-shell] ci: Document how to build the image

[verdre <jonasd src gnome org>]

commit 859927df3971ed199b5d972060594a97cd6a7e07
Author: Florian Müllner <fmuellner gnome org>
Date:   Sat Apr 4 02:35:12 2020 +0200

    ci: Document how to build the image
    
    Updating the CI image is rare enough to not know the involved
    commands by heart, so adding a comment like mutter did a while
    ago makes perfect sense.
    
    https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1177

 .gitlab-ci/Dockerfile | 7 +++++++
 1 file changed, 7 insertions(+)
---
diff --git a/.gitlab-ci/Dockerfile b/.gitlab-ci/Dockerfile
index 6ec6aff9f2..4c7f1097d2 100644
--- a/.gitlab-ci/Dockerfile
+++ b/.gitlab-ci/Dockerfile
@@ -1,3 +1,10 @@
+# Rebuild and push with
+#
+#     cd .gitlab-ci/
+#     podman build --format docker --no-cache -t registry.gitlab.gnome.org/gnome/gnome-shell/extension-ci:v1 
.
+#     podman push registry.gitlab.gnome.org/gnome/gnome-shell/extension-ci:v1
+#
+
 FROM registry.fedoraproject.org/fedora:latest
 
 RUN dnf -y update && dnf -y upgrade && \