Allow unprivileged lxc creation

[wandrewromano]

Is your feature request related to a problem? Please describe.
Proxmox can create unprivileged LXCs that use a separate namespace for uid's and gid's within the container. This can help mitigate container escape issues by mapping to an unprivileged user account. Currently the unprivileged parameter is unset and defaults to false.

Describe the solution you'd like
I think the ideal solution would be a checkbox that can allow a user to choose if an unprivileged container should be created during the build process.

Describe alternatives you've considered
Containers can be manually backed up and restored through Proxmox to change their privileged status.

Thank you for your work on this project.