[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/rspamd/Dockerfile

@@ -6,8 +6,11 @@ ENV LC_ALL C
 
 RUN apt-key adv --fetch-keys http://rspamd.com/apt-stable/gpg.key \
 	&& echo "deb http://rspamd.com/apt-stable/ jessie main" > /etc/apt/sources.list.d/rspamd.list \
-	&& apt-get update \
-	&& apt-get -y install rspamd ca-certificates python-pip
+	&& apt-get update && apt-get install -y \
+		rspamd \
+		ca-certificates \
+		python-pip \
+	&& rm -rf /var/lib/apt/lists/*
 
 RUN echo '.include $LOCAL_CONFDIR/local.d/rspamd.conf.local' > /etc/rspamd/rspamd.conf.local
 
@@ -18,7 +21,7 @@ RUN pip install -U oletools
 
 CMD /usr/bin/rspamd -f -u _rspamd -g _rspamd
 
-RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+RUN rm -rf /tmp/* /var/tmp/*
 
 USER _rspamd