Use only 1 Dockerfile for Windows

Dockerfile.windows

@@ -1,11 +1,14 @@
-FROM golang:nanoserver AS build
+ARG golang=golang:nanoserver
+ARG target=microsoft/nanoserver
+
+FROM $golang AS build
 
 COPY . /code
 WORKDIR /code
 
 RUN go build http.go
 
-FROM microsoft/nanoserver
+FROM $target
 
 COPY --from=build /code/http.exe /http.exe