Build images with genuinetools/img

It will be awesome of gitkube can integrate with something like `img` so that images can be built without mounting the docker sock.

ref:
https://github.com/genuinetools/img
https://blog.jessfraz.com/post/building-container-images-securely-on-kubernetes/

There are still some issue to be merged/closed for this to happen, mentioned in the blog above ^