[Dockerfiles] Replaced deprecated MAINTAINER with LABEL

Used format proposed in
https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

data/Dockerfiles/clamav/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:stretch-slim
-MAINTAINER https://m-ko.de Markus Kosmal <code@cnfg.io>
+LABEL maintainer "https://m-ko.de Markus Kosmal <code@cnfg.io>"
 
 # Debian Base to use
 ENV DEBIAN_VERSION stretch

data/Dockerfiles/dovecot/Dockerfile

@@ -1,6 +1,6 @@
 FROM debian:stretch-slim
 #ubuntu:xenial
-MAINTAINER Andre Peters <andre.peters@servercow.de>
+LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 ENV LC_ALL C

data/Dockerfiles/php-fpm/Dockerfile

@@ -1,5 +1,5 @@
 FROM php:7.1-fpm
-MAINTAINER Andre Peters <andre.peters@servercow.de>
+LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 

data/Dockerfiles/postfix/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:stretch-slim
-MAINTAINER Andre Peters <andre.peters@servercow.de>
+LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 ENV LC_ALL C

data/Dockerfiles/rmilter/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:jessie-slim
-MAINTAINER Andre Peters <andre.peters@servercow.de>
+LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 ENV LC_ALL C

data/Dockerfiles/rspamd/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:jessie-slim
-MAINTAINER Andre Peters <andre.peters@servercow.de>
+LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 ENV LC_ALL C

data/Dockerfiles/sogo/Dockerfile

@@ -1,5 +1,5 @@
 FROM debian:jessie-slim
-MAINTAINER Andre Peters <andre.peters@servercow.de>
+LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 ENV LC_ALL C