TT#49687 Refactor Dockerfile

Use docker best practice - reduce the number of layers.

Sort list of used packages.

Change-Id: Iecc45c9a6f1ac490380b7a19419c64a4580f868e

t/tap-tests-docker/Dockerfile

@@ -5,31 +5,39 @@ FROM docker.mgm.sipwise.com/sipwise-jessie:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT 2019-02-27
-
-RUN echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
-deb https://deb.sipwise.com/autobuild/ internal-jessie main\n" > /etc/apt/sources.list.d/sipwise-internal.list
-
-RUN echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
-deb [arch=amd64] https://debian.sipwise.com/debian jessie-backports main contrib\n" > /etc/apt/sources.list.d/debian-backports.list
+ENV REFRESHED_AT 2020-01-24
 
 RUN echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
+deb https://deb.sipwise.com/autobuild/ internal-jessie main\n" > \
+  /etc/apt/sources.list.d/sipwise-internal.list && \
+  echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
+deb [arch=amd64] https://debian.sipwise.com/debian jessie-backports main contrib\n" > \
+  /etc/apt/sources.list.d/debian-backports.list && \
+  echo "# generated by Dockerfile from jenkins-tap-test-jessie at $(date)\n\
 Explanation: force installing pep8 from jessie-backports\n\
 Package: pep8\n\
 Pin: release n=jessie-backports\n\
 Pin-Priority: 600\n" > /etc/apt/preferences.d/pep8.pref
 
 RUN apt-get update && apt-get install --assume-yes --no-install-recommends \
-  shellcheck pep8 libperl-critic-perl ruby jenkins-debian-glue parallel \
-  file binutils xz-utils && \
+  binutils \
+  file \
+  jenkins-debian-glue \
+  libperl-critic-perl \
+  parallel \
+  pep8 \
+  ruby \
+  shellcheck \
+  xz-utils && \
   apt-get clean
 
 # The package devscripts has a huge list of dependencies and
 # we need checkbashisms script only, providing it manually
-RUN mkdir /tmp/zzz && cd /tmp/zzz && \
-apt-get download devscripts && \
-dpkg --fsys-tarfile devscripts_*.deb | tar -C / -p -xv ./usr/bin/checkbashisms && \
-rm -rf /tmp/zzz
+RUN mkdir /tmp/zzz && \
+  cd /tmp/zzz && \
+  apt-get download devscripts && \
+  dpkg --fsys-tarfile devscripts_*.deb | tar -C / -p -xv ./usr/bin/checkbashisms && \
+  rm -rf /tmp/zzz
 
 COPY t/tap-tests-docker/perlcriticrc /root/.perlcriticrc