build without docker

[justincormack]

(This came from discussion at Edinburgh Docker meetup)

Need to have the ability to build in a container (without Docker socket access). Example is in AWS Codebuild pipeline.

Currently Docker is needed for some output formats, more work needs doing on the linuxkit based outputs but these do work with qemu.

Secondly it is needed to extract image filesystems. We should switch to containerd for these; either using image store, or filesystem as now, or filesystem layers that get mounted later.

[justincormack]

Ongoing work on this in linuxkit