Investigate alignment/intersection with systemd-repart

Let's look at [systemd repart](https://www.freedesktop.org/software/systemd/man/latest/systemd-repart.service.html) and our intersections with it.

A big picture overall thing here is that `bootc install` defaults to having one rootfs and inherently can work on top of any filesystem or block storage. A key thing is that the operating system size is dynamic, not a fixed partition A/B setup.

systemd-repart is definitely oriented towards DDIs and the discoverable partition spec and less so to this path, but it is obviously very general.

I think a baseline pattern that would be neat to support is injecting repart configs into the base container image to define a distinct `/var` partition.

A great first target would be e.g. initializing EBS instance store volumes in AWS, or the NTFS partition in Azure.

(Note, it looks like the service is designed to run from the initrd, so we'll want to document either regenerating or more ideally *appending* to the initrd)

リンクされている systemd-repart のドキュメントと、bootc install および initrd のパスから始めます。repart で独立した /var パーティションを定義し、AWS EBS instance-store または Azure NTFS ボリュームを初期化する方法を比較します。initrd の設定を再生成するのか追記するのかを含む統合方針を決定し、その結果の動作を文書化できれば完了です。

索引モデルが issue の本文から書いたものです。