kola/docker: update test image creation
kola/docker: update test image creation

The previous tar script both printed a warning and was subtly wrong.
Using `docker build` is a much saner way to create this image.
More importantly, the previous method broke when run as a non-root user
due to crazy reasons.

marineam/mantle,marineam/mantle,quantum/coreos-mantle,dm0-/mantle,quantum/coreos-mantle,dm0-/mantle,glevand/coreos--mantle,glevand/coreos--mantle