[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/php-fpm/Dockerfile

@@ -3,14 +3,15 @@ LABEL maintainer "Andre Peters <andre.peters@servercow.de>"
 
 ENV DEBIAN_FRONTEND noninteractive
 
-RUN apt-get update \
-	&& apt-get install -y zlib1g-dev \
+RUN apt-get update && apt-get install -y \
+	zlib1g-dev \
 	libicu-dev \
 	g++ \
 	libidn11-dev \
 	libxml2-dev \
 	redis-tools \
-	mysql-client
+	mysql-client \
+	&& rm -rf /var/lib/apt/lists/*
 
 RUN docker-php-ext-configure intl
 RUN docker-php-ext-install intl pdo pdo_mysql xmlrpc