Windows: add a toolchain image for 10.0.19044.1706 (21H2) #302
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Add rules for building a docker image for the toolchain build at
10.0.19044.1706 (21H2). As the base image for the release is not
available, use a slightly older release which should be compatible
to avoid any issues due to kernel mismatches (the newer image will
not run). This allows building the toolchain in a docker image with
the latest stable release of Windows 10.