dockerfile: merge RUN layers

Dockerfile

@@ -5,19 +5,15 @@ COPY ./devolo-cockpit-v5-1-6-2-linux.run /devolo-cockpit-setup
 RUN dpkg --add-architecture i386 \
     && apt-get update \
     && apt-get install --no-install-recommends --yes lzma xz-utils \
-    && echo TODO merge
-RUN apt-get install -y x11-common dbus ubuntu-mono
-RUN apt-get install -y gcc-8-base:i386 libc6:i386 libx11-6:i386
-RUN apt-get install -y libgtk2.0-0:i386 libgcrypt20:i386 libstdc++6:i386 libxml2:i386
-RUN apt-get install -y libnss3:i386
-RUN apt-get install -y libbz2-1.0:i386 libxaw7:i386
-RUN DEBIAN_FRONTEND=noninteractive /devolo-cockpit-setup \
+    && DEBIAN_FRONTEND=noninteractive /devolo-cockpit-setup \
     && apt-get purge --autoremove --yes xz-utils \
     && dpkg --listfiles devolo-dlan-cockpit \
-    && rm -rf /devolo-cockpit-setup /var/lib/apt/lists/*
-RUN rm /etc/apt/sources.list.d/devolo-updates.list
+    && rm -rf /devolo-cockpit-setup /var/lib/apt/lists/* \
+    && rm /etc/apt/sources.list.d/devolo-updates.list
 
-RUN apt-get update && apt-get install --no-install-recommends --yes strace
+RUN apt-get update \
+    && apt-get install --no-install-recommends --yes strace \
+    && rm -rf /var/lib/apt/lists/*
 
 # gui: strace -e trace=!recvmsg,poll,futex,mprotect,semop,writev -f /opt/devolo/dlancockpit/bin/dlancockpit-run.sh
 CMD ["strace", "-e", "trace=!_newselect,clock_gettime", "-f", "/usr/bin/devolonetsvc"]