Update Dockerfile

Signed-off-by: Teddysun <[email protected]>

docker/l2tp/Dockerfile

@@ -2,7 +2,7 @@
 # Copyright (C) 2018 Teddysun <[email protected]>
 
 FROM debian:stretch
-MAINTAINER Teddysun <[email protected]>
+LABEL maintainer="Teddysun <[email protected]>"
 
 RUN set -ex \
     && printf "deb http://deb.debian.org/debian sid main" > /etc/apt/sources.list.d/sid.list \