Converting a virtual machine and the installed applications on the VM to a docker image

I have installed Ubuntu 20.04 in the virtual box and then made a .tar file of the whole virtual box and then moved it onto docker and build it as an image in windows 10. When I am trying to run this image I am getting the below error
(Error invoking remote method ‘docker-run-container’: Error: (HTTP code 400) unexpected - No command specified)

To build the docker image I have used the below docker file script

FROM scratch
ADD img.tar /

I have referred to the book Docker in practice by Ian Miell, chapter-3, From VM to Docker for the above process.