Add support for docker.io virtualization.
Adding the possibility to use docker as a virt method makes things easier when using autopkgtest on a gitlab-ci pipeline.
I brought this changes from https://github.com/chriskuehl/autopkgtest
Sincerely, I don't know how is the best way to do that.
Reading the issue #747909 I found that the repo owner (chriskuehl) participated but it was never merged to this repository.
Edited by Simon McVittie