Configure rootfs export to not include `rootfs/` in the output .tar.gz

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#### Description

I am using Packer to provision container templates for a Proxmox VE instance, and Proxmox expects that the input `.tar.gz` file containing the rootfs be "direct", for lack of a better word. That is, `/bin/`, `/dev/`, and the like should be at the top level of the archive rather than contained inside of a `/rootfs/` folder.

Obviously I can work around this by doing a `post-process` step, but it's a bit cumbersome. I'd like a way to configure the export step which lets me solve this use case. (I'm assuming this needs to be more general than something to solve this very specific case, but maybe not.)

#### Use Case(s)

Proxmox containers is my current use case, although I'd not be surprised to learn there is other LXC-based infrastructure that has this same problem.

#### Potential configuration

Not really sure. Maybe adding a key like this to the builder config?

```
archive_directory = $containerDir/rootfs
```

That syntax definitely needs some work.

#### Potential References