For Go 1.17+, move the Windows install to C:\Program Files\Go

This matches Go upstream's change to the same, and allows GOPATH on 1.17+ to move to C:\go (although with the caveat that GOPATH may not actually be useful for much longer).

See https://golang.org/cl/283600 for where this was changed in the official Go installer.

Author: tianon

1.15/windows/nanoserver-1809/Dockerfile

@@ -5,18 +5,39 @@ SHELL ["cmd", "/S", "/C"]
 # no Git installed (intentionally)
 #  -- Nano Server is "Windows Slim"
 
-# ideally, this would be C:\go to match Linux a bit closer, but C:\go is the recommended install path for Go itself on Windows
+{{
+	def install_directory:
+		if [ "1.15", "1.16" ] | index(env.version) then
+			"C:\\go"
+		else
+			"C:\\Program Files\\Go"
+		end
+-}}
+{{ if install_directory == "C:\\go" then ( -}}
+# ideally, this would be C:\go to match Linux a bit closer, but C:\go was the default install path for Go itself on Windows
 ENV GOPATH C:\\gopath
+# (https://golang.org/cl/283600)
+{{ ) else ( -}}
+# for 1.17+, we'll follow the (new) Go upstream default for install (https://golang.org/cl/283600), which frees up C:\go to be the default GOPATH and thus match the Linux images more closely (https://github.com/docker-library/golang/issues/288)
+ENV GOPATH C:\\go
+# HOWEVER, please note that it is the Go upstream intention to remove GOPATH support entirely: https://blog.golang.org/go116-module-changes
+{{ ) end -}}
 
 # PATH isn't actually set in the Docker image, so we have to set it from within the container
 USER ContainerAdministrator
-RUN setx /m PATH "%GOPATH%\bin;C:\go\bin;%PATH%"
+RUN setx /m PATH "%GOPATH%\bin;{{ install_directory }}\bin;%PATH%"
 USER ContainerUser
 # doing this first to share cache across versions more aggressively
 
 ENV GOLANG_VERSION {{ .version }}
 
-COPY --from=golang:{{ .version }}-windowsservercore-{{ env.windowsRelease }} C:\\go C:\\go
+# Docker's Windows path parsing is absolutely *cursed*; please just trust me on this one -Tianon
+COPY --from=golang:{{ .version }}-windowsservercore-{{ env.windowsRelease }} {{
+	install_directory
+	| gsub("\\\\"; "\\\\")
+	| [ . , . ]
+	| @json
+}}
 RUN go version
 
 WORKDIR $GOPATH