Add Dockerfile to run packer validate in Windows container

Dockerfile

+FROM mcr.microsoft.com/windows-insider:10.0.17744.1001 AS makeiso
+WORKDIR C:/source
+COPY . .
+RUN powershell -NoProfile -ExecutionPolicy unrestricted -file make_unattend_iso.ps1
+
+FROM mcr.microsoft.com/windowsservercore-insider:10.0.17744.1001
+ENV chocolateyUseWindowsCompression false
+
+RUN powershell -NoProfile -ExecutionPolicy unrestricted -Command \
+    iex ((new-object net.webclient).DownloadString('https://chocolatey.org/install.ps1')); \
+    choco feature disable --name showDownloadProgress
+RUN choco install -y packer
+RUN powershell -Command Install-WindowsFeature Hyper-V-PowerShell
+COPY --from=makeiso C:/source C:/source
+WORKDIR C:/source
+RUN powershell -File test.ps1