ci: update apt-get handling to follow latest guidelines

The docker documentation suggests that all the apt-get operations should
be done in a single RUN command.

Additionally it mentions that explicit invocation of apt-get clean is not
needed, and one should rm -rf the local apt cache.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Elie Tournier <elie.tournier@collabora.com>

ci/Dockerfile

@@ -21,8 +21,8 @@ RUN echo 'exit 101' >> /usr/sbin/policy-rc.d
 RUN chmod +x /usr/sbin/policy-rc.d
 
 RUN echo deb-src http://deb.debian.org/debian testing main >> /etc/apt/sources.list
-RUN apt-get update
-RUN apt-get -y install ca-certificates && \
+RUN apt-get update && \
+    apt-get -y install ca-certificates && \
     apt-get -y install --no-install-recommends \
       autoconf \
       busybox \
@@ -74,16 +74,14 @@ RUN apt-get -y install ca-certificates && \
       xserver-xorg-core \
       xterm \
       xvfb \
-      zlib1g-dev
-
-RUN apt-get -y build-dep --no-install-recommends \
+      zlib1g-dev && \
+    apt-get -y build-dep --no-install-recommends \
       libdrm \
       mesa \
       piglit \
-      virglrenderer
-
-RUN apt-get -y remove valgrind && \
-    apt-get clean
+      virglrenderer && \
+    apt-get -y remove valgrind && \
+    rm -rf /var/lib/apt/lists/*
 
 RUN pip3 install meson