Exporting unprivileged LXC containers fails

_This issue was originally opened by @thedrow as hashicorp/packer#6545. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

When the export step is run, it tries to tar the rootfs.
Unfortunately I failed to notice before that it failed due to some of the files in the rootfs belonging to the container's root.
It seems that exporting the container requires us to run sudo. When running sudo you get a prompt which you can't fill since we're not listening to input.
Not sure how to resolve this correctly.