Windows: Fix long path handling for docker build

Signed-off-by: Stefan J. Wernli <swernli@microsoft.com>

longpath/longpath.go

@@ -0,0 +1,21 @@
+// longpath introduces some constants and helper functions for handling long paths
+// in Windows, which are expected to be prepended with `\\?\` and followed by either
+// a drive letter, a UNC server\share, or a volume identifier.
+
+package longpath
+
+import (
+	"strings"
+)
+
+// Prefix is the longpath prefix for Windows file paths.
+const Prefix = `\\?\`
+
+// AddPrefix will add the Windows long path prefix to the path provided if
+// it does not already have it.
+func AddPrefix(path string) string {
+	if !strings.HasPrefix(path, Prefix) {
+		path = Prefix + path
+	}
+	return path
+}