Bug#813007: Fix container detection (running-in-container is an Ubuntu-ism)

[Ian Campbell]

On Thu, 2016-01-28 at 14:55 +0100, Martin Pitt wrote:
> grub2 currently tries to call running-in-container in its postinst and
> zz-update-grub. This script is an Ubuntu-ism and going away (see
> https://launchpad.net/bugs/1539016), and thus container detection does
> not work under Debian.
> 
> Please use systemd-detect-virt --container instead, which works under
> any init system (despite the name).

Will it always be installed or is a Depends needed?