Windows image build times are unnecessarily slow

[lbussell]

On Windows, ImageBuilder is not ran inside a container. We use containers only as a means to shuttle around the ImageBuilder executable, extract it, and run natively on Windows.

Analysis of a recent ImageBuilder build stage

~	Win1809	Win2022	Combined
Image pull	7m 25s	31s	7m 56s
Image build	1m 28s	1m 48s	3m 16s
Image push	4m 41s	16s	4m 58s
Total	13m 35s	2m 35s	16m 10s

Only 20% of combined Windows ImageBuilder build time is the actual docker build. The other 80% is network I/O — pulls (49%) and pushes (31%) — dominated almost entirely by the Win1809 leg's servercore:ltsc2016 image dependency.

[lbussell]

[Triage] We spend a lot of build time shuttling around the servercore:ltsc2016 image in the Win1809 build leg (it builds both ltsc2016 and 1809 images in the same build). 1809 images and onwards use nano server, but server 2016 predates nano server.

We can never use docker-in-docker on server 2016, so even if using imagebuilder as a dotnet tool doesn't work out (#2007), we should find a way to avoid pulling the entire server 2016 image.

Unfortunately there is no such thing as a FROM scratch image without BuildKit on Windows. Maybe there's another, smaller alternative we could use.