[Discussion] Service units and assemble for managed distroboxes

### Is your feature request related to a problem? Please describe.
For projectbluefin.io we're shooting for a "managed toolbox" experience. We'd like the container to be set up for the user automatically, including regular updates, etc. We'd also like to fully destroy the container and rebuild it from scratch on a regular interval.

We use a volume mount to track the package manager's state so that the container is refreshed without the user losing their packaging state. This led to more discussions and I'm capturing them here:

### Describe the solution you'd like

89luca89: I was thinking about `distrobox assemble generate foo` that would generate the assemble file from an existing unmanaged container

Then one could add a `distrobox generate systemd` or something on those lines, that would then create a unit file to recreate it at boot This would make it compatible with docker, podman, lilipod (and future other container managers)

This would also be useful to do "snapshots" of a container, so the assemble file would recreate it as it was in a specific time.

M2: Since everything used to create the container is in the `podman inspect` for the existing container, this would be parsing. Are we limited to shell or could we use jq for this functionality?

Generate systemd sounds great and should be a straightforward implenentation. Should just need the container name for making the unit file.

89luca89: The idea behind an assemble generate would be to "save/snapshot" an unmanaged container, so one where you manually installed/removed stuff, so only parsing the inspect won't be enough but yes, ideally I'd use a go-template string for the --format flag of podman inspect (or docker)

Jq or Yq (for yaml) are useful, but that would become an external dependency of the tool, which would be a first, as for now dbox only needs a posix shell and a container manager.

### Describe alternatives you've considered

We're using podman's quadlets feature to do this: https://github.com/ublue-os/toolboxes/tree/main/quadlets/bluefin-cli

And here's the associated one-shot and assemble: https://github.com/ublue-os/toolboxes/tree/main/systemd/bluefin-cli

查看現有的 'distrobox assemble' 命令以及對 'podman inspect' 輸出的解析。此功能涉及從現有容器產生 assemble 檔案並建立 systemd 單元檔案。首先檢查 'distrobox assemble' 的程式碼以及 'podman inspect' 整合。查看連結範例中如何使用 quadlet。目標是產生一個能夠對容器狀態建立快照並自動化其重新建立的命令。

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