vendor: add buildkit

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/moby/buildkit/util/system/path_windows.go

@@ -0,0 +1,37 @@
+// +build windows
+
+package system
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+// DefaultPathEnv is deliberately empty on Windows as the default path will be set by
+// the container. Docker has no context of what the default path should be.
+const DefaultPathEnv = ""
+
+// CheckSystemDriveAndRemoveDriveLetter verifies and manipulates a Windows path.
+// This is used, for example, when validating a user provided path in docker cp.
+// If a drive letter is supplied, it must be the system drive. The drive letter
+// is always removed. Also, it translates it to OS semantics (IOW / to \). We
+// need the path in this syntax so that it can ultimately be contatenated with
+// a Windows long-path which doesn't support drive-letters. Examples:
+// C:			--> Fail
+// C:\			--> \
+// a			--> a
+// /a			--> \a
+// d:\			--> Fail
+func CheckSystemDriveAndRemoveDriveLetter(path string) (string, error) {
+	if len(path) == 2 && string(path[1]) == ":" {
+		return "", fmt.Errorf("No relative path specified in %q", path)
+	}
+	if !filepath.IsAbs(path) || len(path) < 2 {
+		return filepath.FromSlash(path), nil
+	}
+	if string(path[1]) == ":" && !strings.EqualFold(string(path[0]), "c") {
+		return "", fmt.Errorf("The specified path is not on the system drive (C:)")
+	}
+	return filepath.FromSlash(path[2:]), nil
+}