Identify imported real ubuntu image as docker container

I have the following problem: I have to import an existing ubuntu system as a docker container. I have created a tar from the original system and imported with docker import successfully. The image is running, but when I do a system update grub2 installation fails because grub can not identfy my image as a container.

systemd-detect-virt --container returns none instead of docker

How can I make the container identfy itself as a docker container, what package do I have to install?

Thanks a lot for any suggestions!

Found the solution: there must be a file

which just contains the string

This solves the issue with grub2 as well.