Accepts a docker buildx --output spec and routes the exporter result to a
Windows destination. The client parses the spec using buildx's CSV grammar and
picks one of three paths: dest=- relays the build process's stdout straight to
the caller's handle, a real file destination is written in place through a
read-write virtiofs mount of its parent directory, and everything else (type=image,
type=cacheonly) is forwarded verbatim and stays in the VM.
Directory exporters (type=local, or oci/docker with tar=false) are rejected at
parse time, since a Linux directory tree cannot be materialized faithfully on a
Windows destination. Writing a binary exporter stream to an interactive console
is rejected as well.
Adds unit tests covering the spec grammar and rejection cases, plus e2e coverage
for each exporter type.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
