hack/ci/windows.ps1: build busybox from local Dockerfile

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/busybox/Dockerfile

@@ -1,18 +1,19 @@
-# Source: http://frippery.org/busybox/
+# Source: https://frippery.org/busybox/
 # This Dockerfile builds a (32-bit) busybox images which is suitable for
 # running many of the integration-cli tests for Docker against a Windows
 # daemon. It will not run on nanoserver as that is 64-bit only.
 #
+# Based on https://github.com/jhowardmsft/busybox
 # John Howard (IRC jhowardmsft, Email john.howard@microsoft.com)
 #
 # To build: docker build -t busybox .
 # To publish: Needs someone with publishing rights
-#
-# http://github.com/jhowardmsft/busybox
+ARG WINDOWS_BASE_IMAGE=mcr.microsoft.com/windows/servercore
+ARG WINDOWS_BASE_IMAGE_TAG=ltsc2019
 
-FROM microsoft/windowsservercore
+FROM ${WINDOWS_BASE_IMAGE}:${WINDOWS_BASE_IMAGE_TAG}
 RUN mkdir C:\tmp && mkdir C:\bin
 ADD http://frippery.org/files/busybox/busybox.exe /bin/
 RUN setx /M PATH "C:\bin;%PATH%"
 RUN powershell busybox.exe --list ^|%{$nul = cmd /c mklink C:\bin\$_.exe busybox.exe}
-CMD ["sh"]
+CMD ["sh"]