Add BuildArg key to quadlets

[givensuman]

This addresses #27065, adding a BuildArg key to quadlet .build files. I believe I correctly integrated this into existing tests with this file.

Does this PR introduce a user-facing change?

Add BuildArg key to quadlet `.build` files which equates to `podman build --build-arg` flag.

[ygalblum]

Thanks for the PR.
In addition, the new key should be documented in docs/source/markdown/podman-systemd.unit.5.md

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[baude]

our linters found some issues as well which is why CI failed. assuming you have podman installed in $PATH, a make validatepr should run them for you before you check things in.

[baude]

LGTM, @ygalblum if you are happy with this, can you also give a LGTM?

[packit-as-a-service]

Cockpit tests failed for commit b4d81c0. @martinpitt, @jelly, @mvollmer please check.

[ygalblum]

LGTM
@baude I'm not officially approving the PR, because @TomSweeneyRedHat has an unresolved comment

[packit-as-a-service]

[NON-BLOCKING] Packit jobs failed. @containers/packit-build please check. Everyone else, feel free to ignore.

[mheon]

Both test failures look like flakes. Restarted.

[mheon]

/approve
LGTM

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: givensuman, mheon

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[ygalblum]

@TomSweeneyRedHat do we have your thumbs up for the doc fixes?