docs: added simple extra_args example

[akiross]

This PR adds a simple example of how to use extra_args in podman image build.

As discussed here I think the example could be useful.

[sshnaidm]

Please sign your commit: git commit -s, it's must in all repos in this org.
You can do git commit -s --amend and git push --force just to add the signature.
All commits should be signed.

[sshnaidm]

And please rebase on latest master, thanks!

[sshnaidm]

@akiross hi, can you please update your patch with rebase and signing so I can merge it?
Thanks!

[akiross]

@sshnaidm sorry for being late, but I had some serious issues in the way. Thank you for taking care of this, though.