Update Dockerfile.wintest

Author: alex-codefresh

Dockerfile.wintest

@@ -1,7 +1,7 @@
 # escape=`
 
 # Installer image
-FROM microsoft/windowsservercore:1709 AS installer-env
+FROM microsoft/windowsservercore:1803 AS installer-env
 
 SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue';"]
 
@@ -21,7 +21,7 @@ RUN Invoke-WebRequest $Env:DOTNET_SDK_DOWNLOAD_URL -OutFile dotnet.zip; `
 
 
 # SDK image
-FROM microsoft/nanoserver:1709
+FROM microsoft/nanoserver:1803
 
 COPY --from=installer-env ["dotnet", "C:\\Program Files\\dotnet"]