[Dockerfiles] Do not persist DEBIAN_FRONTEND=noninteractive in images

According to https://github.com/moby/moby/issues/4032#issuecomment-163689851
(and some other comments in the issue) it's not recommended to set
`DEBIAN_FRONTEND` via `ENV` in a Dockerfile.  `ARG` has the same
effect at build time but does not change `DEBIAN_FRONTEND` in the
final image, so I switched to it.  It should also work to remove it
completely.

data/Dockerfiles/clamav/Dockerfile

@@ -3,12 +3,13 @@ LABEL maintainer "https://m-ko.de Markus Kosmal <code@cnfg.io>"
 
 # Debian Base to use
 ENV DEBIAN_VERSION stretch
+ARG DEBIAN_FRONTEND=noninteractive
 
 # initial install of av daemon
 RUN echo "deb http://http.debian.net/debian/ $DEBIAN_VERSION main contrib non-free" > /etc/apt/sources.list && \
 	echo "deb http://http.debian.net/debian/ $DEBIAN_VERSION-updates main contrib non-free" >> /etc/apt/sources.list && \
 	echo "deb http://security.debian.org/ $DEBIAN_VERSION/updates main contrib non-free" >> /etc/apt/sources.list && \
-	apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends \
+	apt-get update && apt-get install -y -qq --no-install-recommends \
 		clamav-daemon \
 		clamav-freshclam \
 		libclamunrar7 \

data/Dockerfiles/dovecot/Dockerfile

@@ -2,7 +2,7 @@ FROM debian:stretch-slim
 #ubuntu:xenial
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
-ENV DEBIAN_FRONTEND noninteractive
+ARG DEBIAN_FRONTEND=noninteractive
 ENV LC_ALL C
 ENV DOVECOT_VERSION 2.2.29.1
 ENV PIGEONHOLE_VERSION 0.4.18

data/Dockerfiles/php-fpm/Dockerfile

@@ -1,7 +1,7 @@
 FROM php:7.1-fpm
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
-ENV DEBIAN_FRONTEND noninteractive
+ARG DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y \
 	g++ \

data/Dockerfiles/postfix/Dockerfile

@@ -1,7 +1,7 @@
 FROM debian:stretch-slim
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
-ENV DEBIAN_FRONTEND noninteractive
+ARG DEBIAN_FRONTEND=noninteractive
 ENV LC_ALL C
 
 RUN dpkg-divert --local --rename --add /sbin/initctl \

data/Dockerfiles/rmilter/Dockerfile

@@ -1,7 +1,7 @@
 FROM debian:jessie-slim
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
-ENV DEBIAN_FRONTEND noninteractive
+ARG DEBIAN_FRONTEND=noninteractive
 ENV LC_ALL C
 
 RUN apt-key adv --fetch-keys http://rspamd.com/apt-stable/gpg.key \

data/Dockerfiles/rspamd/Dockerfile

@@ -1,7 +1,7 @@
 FROM debian:jessie-slim
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
-ENV DEBIAN_FRONTEND noninteractive
+ARG DEBIAN_FRONTEND=noninteractive
 ENV LC_ALL C
 
 RUN apt-key adv --fetch-keys http://rspamd.com/apt-stable/gpg.key \

data/Dockerfiles/sogo/Dockerfile

@@ -1,7 +1,7 @@
 FROM debian:jessie-slim
 LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
-ENV DEBIAN_FRONTEND noninteractive
+ARG DEBIAN_FRONTEND=noninteractive
 ENV LC_ALL C
 ENV GOSU_VERSION 1.9