CI: do not rebuild the image for regular MR or pushes

We better not rebuild the image in regular operations unless there is a
strong need for it.
We can however set up a scheduled pipeline to rebuild the images once
a week or once a month in the upstream repo, and the forks will fetch
those new images when they need.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
(cherry picked from commit 51e66ab9)