Convert docker file to flist

[andrewayoub]

Currently you can convert a docker image to flist but this image needs to be published first, I think It's a good idea to be able to convert a docker file directly into an flist without needing to publish the produced image first.

this will be very useful after we launch the grid and people start to use zos nodes they can migrate their docker file based projects to flists and use it directly in containers.