driver/docker: convert host bind path to os native

relative mounting can be specified using backslashes or forward slashes.
so no prior knowledge of host OS is needed for relative volumes mounting

drivers/docker/utils.go

@@ -237,7 +237,7 @@ func parseVolumeSpecWindows(volBind string) (hostPath string, containerPath stri
 		return "", "", "", fmt.Errorf("not <src>:<destination> format")
 	}
 
-	hostPath = parts[0]
+	hostPath = filepath.FromSlash(parts[0])
 	containerPath = parts[1]
 
 	if len(parts) > 2 {