integration: download busybox-w32 from a Docker image

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

contrib/busybox/Dockerfile

@@ -12,14 +12,18 @@ ARG WINDOWS_BASE_IMAGE=mcr.microsoft.com/windows/servercore
 ARG WINDOWS_BASE_IMAGE_TAG=ltsc2022
 ARG BUSYBOX_VERSION=FRP-3329-gcf0fa4d13
 
+# Busybox w32 binary from https://frippery.org/files/busybox/ available
+# as a Docker image: https://github.com/crazy-max/docker-busybox-w32
+FROM crazymax/busybox-w32:${BUSYBOX_VERSION} AS busybox
+
 # Checksum taken from https://frippery.org/files/busybox/SHA256SUM
 ARG BUSYBOX_SHA256SUM=bfaeb88638e580fc522a68e69072e305308f9747563e51fa085eec60ca39a5ae
 
 FROM ${WINDOWS_BASE_IMAGE}:${WINDOWS_BASE_IMAGE_TAG}
 RUN mkdir C:\tmp && mkdir C:\bin
 ARG BUSYBOX_VERSION
 ARG BUSYBOX_SHA256SUM
-ADD https://frippery.org/files/busybox/busybox-w32-${BUSYBOX_VERSION}.exe /bin/busybox.exe
+COPY --from=busybox /busybox.exe /bin/busybox.exe
 RUN powershell \
     if ((Get-FileHash -Path /bin/busybox.exe -Algorithm SHA256).Hash -ne $Env:BUSYBOX_SHA256SUM) { \
         Throw \"Checksum validation failed\" \