docs: Add doc about build rpm-ostree+autosd

[dougsland]

Myself and Eric we spend a bunch of time to create rpm-ostree RPM for later inject it into autosd image. To avoid others to spend time in such effort, here a documentation.

[openshift-ci]

Hi @dougsland. Thanks for your PR.

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[cgwalters]

FTR I have no problem just merging this as is (it's better than not having it), but I think just a few cleanups would help.

[cgwalters]

The other really popular thing in this space besides toolbx is vscode devcontainer but it's all proprietary.

[dougsland]

FTR I have no problem just merging this as is (it's better than not having it), but I think just a few cleanups would
help.

I agree but If doesn't fit well, we can always use copy and past intogist githuburl.

[dougsland]

@cgwalters I am open to do changes or improvements as you guys request, no problem at all. The next thing is do a full test against the changes from #4721 with this full environment working.

[ericcurtin]

This generally LGTM, whether this belongs in Automotive documentation or here I don't mind.

Some of the techniques here are debatable, I think everyone may do a couple of these things differently.

One problem we had was the rpm generated on @dougsland 's machine had a lower version than on CentOS Stream 9 (this was the goal, to just generate an rpm with a higher version than in AutoSD).

So when we did final testing in an osbuild run, it wasn't getting pulled in.

The final test we always do is an osbuild run because that's the way rpm-ostree sets up things in the real-world case.

Although I wonder would a git fetch --tags have fixed the version number thing.

[ericcurtin]

I do think a Containerfile and a simple shell script for local development to generate rpms would be helpful.

[dougsland]

I believe I covered all requestes @cgwalters @ericcurtin