move required build files to configmap for clarity

[cheesesashimi]

- What I did

The Containerfile template has a lot of indirection contained within it which
is very difficult to reason about and debug, especially when templating is
involved. To make this easier, I extracted as much as possible from the various
build scripts and pushed those scripts into the Containerfile ConfigMap via the
buildrequest/assets directory.

- How to verify it

Everything should continue to work as-is. This should be tested against a proxy-enabled cluster just to validate that these changes did not have any unintended side-effects.

- Description for the changelog
Move required build files to ConfigMap for clarity

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Skipping CI for Draft Pull Request.
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You can still manually trigger a test run with /test all

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[RishabhSaini]

Having looked at the changes to the buildah-build.sh script and the inner-script, they both correctly utilize users.
LGTM

[RishabhSaini]

I had a question of how install-extensions.sh was getting called and with what packages as arguments?

[RishabhSaini]

Ok got it here:
https://github.com/openshift/machine-config-operator/pull/4847/files#diff-5af9b9663238f01bb6149d21108a05554182600a317f9a2496f5d494ed558fdeR35

[cheesesashimi]

I had a question of how install-extensions.sh was getting called and with what packages as arguments?

Those are the resolved packaegs that we get from the various extensions that someone wishes to install. Previously, we were just passing them to rpm-ostree install. However, we have a need to perform some post-installation actions on a per-package basis. While it would be great to do it all in the Containerfile without needing additional files and scripts, maintaining that is complicated, to say the least.