Don't join rootfs if path already prefixed by it

In docker we evaluate all symlinks im path to check that they  not escaping
from rootfs and that gives us full path to mount.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

rootfs_linux.go

@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"time"
 
@@ -86,9 +87,13 @@ func setupRootfs(config *configs.Config, console *linuxConsole) (err error) {
 
 func mount(m *configs.Mount, rootfs, mountLabel string) error {
 	var (
-		dest = filepath.Join(rootfs, m.Destination)
+		dest = m.Destination
 		data = label.FormatMountLabel(m.Data, mountLabel)
 	)
+	if !strings.HasPrefix(dest, rootfs) {
+		dest = filepath.Join(rootfs, dest)
+	}
+
 	switch m.Device {
 	case "proc":
 		if err := os.MkdirAll(dest, 0755); err != nil && !os.IsExist(err) {