docker: Clean apt caches after installation

Combining both installation steps and cleaning caches afterwards saves
about 12 MB.

At this chances reformat to have only one command per line.

Suggested-by: Mustafa Yuecel <mustafa.yuecel@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -3,18 +3,22 @@
 FROM debian:8.8
 
 ENV LOCALE=en_US.UTF-8
-RUN apt-get update && apt-get install -y locales && \
+RUN apt-get update && \
+    apt-get install -y locales && \
     sed -i -e "s/# $LOCALE.*/$LOCALE UTF-8/" /etc/locale.gen && \
-    dpkg-reconfigure --frontend=noninteractive locales
-
-RUN apt-get -y install gawk wget git-core diffstat unzip \
+    dpkg-reconfigure --frontend=noninteractive locales && \
+    apt-get -y install gawk wget git-core diffstat unzip \
                        texinfo gcc-multilib build-essential \
                        chrpath socat cpio python python3 \
                        tar bzip2 curl dosfstools mtools parted \
                        syslinux tree python3-pip bc python3-yaml \
-                       lsb-release
+                       lsb-release && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
+
 RUN wget -nv -O /usr/bin/gosu "https://github.com/tianon/gosu/releases/download/1.10/gosu-amd64" && \
     chmod +x /usr/bin/gosu
+
 COPY . /kas
 RUN pip3 install /kas