windows: support basic cross-platform builds from windows

[profnandaa]

Description

Currently, you can build Windows images from Linux (for dockerfiles without RUN commands).
We need to reciprocate this too on Windows, being able to build basic Linux images from Windows.

This can be helpful for cross-platform software packaging.

For example:

FROM alpine

COPY / /

Build command:

> buildctl build --frontend dockerfile.v0 --local context=. --local dockerfile=.§
--output type=image,name=docker.io/profnandaa/cross-build,push=false `
--progress plain --no-cache `
--opt platform=linux/amd64

You get this error:

#7 [2/2] COPY / /
#7 ERROR: hcsshim::ProcessBaseLayer 
\\?\C:\ProgramData\containerd\root\io.containerd.snapshotter.v1.windows\snapshots\1060: 
The system cannot find the path specified.