GitLab

Add a warning when building an image using "onbuild" tags.
Rework the documentation to make "onbuild" less documented and not the "first
example".

We deprecated onbuild images because too much magic occur with them, and
working with ARG/ENV on onbuild images is complicated.

They will probably be planned for deprecation on docker itself: https://github.com/docker-library/official-images/issues/2076