Add support for umask when exec container

Signed-off-by: WangXiaoSong <wang.xiaosong1@zte.com.cn>

libcontainer/setns_init_linux.go

@@ -59,6 +59,10 @@ func (l *linuxSetnsInit) Init() error {
 			return err
 		}
 	}
+	if l.config.Config.Umask != nil {
+		unix.Umask(int(*l.config.Config.Umask))
+	}
+
 	if err := selinux.SetExecLabel(l.config.ProcessLabel); err != nil {
 		return err
 	}