[Dockerfiles] Used best practices for apt-get

See https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#apt-get

- Replaced `-y install` with `install -y` (unification)

- Put every package on its own line

- Moved `rm -rf /var/lib/apt/lists/*` in the same `RUN` statement as `apt-get`

- Removed unnecessary `apt-get clean`
  See 03e2923e42/contrib/mkimage/debootstrap (L82-L105)

data/Dockerfiles/clamav/Dockerfile

@@ -8,14 +8,12 @@ ENV DEBIAN_VERSION stretch
 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 --no-install-recommends -y -qq \
+	apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y -qq --no-install-recommends \
 		clamav-daemon \
 		clamav-freshclam \
 		libclamunrar7 \
-		curl && \
-	apt-get clean && \
-	rm -rf /var/lib/apt/lists/*
+		curl \
+	&& rm -rf /var/lib/apt/lists/*
 
 # initial update of av databases
 COPY dl_files.sh /dl_files.sh