Dockerfile.tmpl: merge apt-get lines

As recommended by [1], "Always combine RUN apt-get update with apt-get
install in the same RUN statement". Also, ditch "apt-get clean", as it
seems to be useless.

[1] https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/

https://travis-ci.org/kolyshkin/criu/builds/173168198
Cc: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/build/Dockerfile.tmpl

 ARG CC=gcc
 
-RUN apt-get clean
-
-RUN apt-get update
-
-RUN apt-get install -y \
+RUN apt-get update && apt-get install -y \
                 build-essential \
                 protobuf-c-compiler \
                 libprotobuf-c0-dev \