Comment 7 for bug 1751249

> For a possibly-reusable example of this from the joule images, see:
>
> git+ssh://git.launchpad.net/~vorlon/tuchuck/+git/uc16-build-script

This script is exactly what we want to get rid of for the upcoming
customers and what triggered the work making me hit this bug/missing-feature
altogether.

For production images a simple "ubuntu-image /path/to/model" should
DTRT for the customer, there should not be any wrapper scripts needed
and there should not be any files injected that do not come from a snap
(like you would not ship randomly injected files in a desktop iso that
are not part of a .deb or postinst script shipped in a .deb).