TT#21905 Switch ngcpcfg Dockerfile to Debian stretch

We need to use python3-junitxml from our own repository,
so enable release-trunk-stretch

Change-Id: I6fc31245854a0cc8fe5fff03d351c451fac693c6

t/Dockerfile

@@ -1,19 +1,24 @@
-# DOCKER_NAME=ngcpcfg-jessie
-FROM docker.mgm.sipwise.com/sipwise-jessie:latest
+# DOCKER_NAME=ngcpcfg-stretch
+FROM docker.mgm.sipwise.com/sipwise-stretch:latest
 
 # Important! Update this no-op ENV variable when this Dockerfile
 # 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 2017-06-21
+ENV REFRESHED_AT 2017-09-22
 
 RUN apt-get update
 
 # sourcecode test dependencies
-RUN apt-get install --assume-yes netcat libhash-merge-perl libtemplate-perl libyaml-libyaml-perl libyaml-tiny-perl libyaml-perl 
+RUN apt-get install --assume-yes netcat libhash-merge-perl libtemplate-perl libyaml-libyaml-perl libyaml-tiny-perl libyaml-perl
 # misc stuff for execution/debugging/devel
 RUN apt-get install --assume-yes fakeroot git lsof make strace
-# test execution
+# test execution; we need the backport of python3-junitxml from our own
+# repository since it's not part of Debian/stretch
+# files that get-code generates
+COPY t/sources.list.d/builddeps.list /etc/apt/sources.list.d/
+COPY t/sources.list.d/preferences /etc/apt/preferences.d/
+RUN apt-get update
 RUN apt-get install --assume-yes python3-pytest python3-junitxml
 
 RUN echo './t/testrunner' >>/root/.bash_history
@@ -24,12 +29,12 @@ WORKDIR /code/
 # Instructions for usage
 # ----------------------
 # When you want to build the base image from scratch (jump to the next section if you don't want to build yourself!):
-# % docker build --tag="ngcpcfg-jessie" .
-# % docker run --rm -i -t -v $(pwd)/..:/code:rw ngcpcfg-jessie:latest bash
+# % docker build --tag="ngcpcfg-stretch" .
+# % docker run --rm -i -t -v $(pwd)/..:/code:rw ngcpcfg-stretch:latest bash
 #
 # Use the existing docker image:
-# % docker pull docker.mgm.sipwise.com/ngcpcfg-jessie
-# % docker run --rm -i -t -v $(pwd)/..:/code:rw docker.mgm.sipwise.com/ngcpcfg-jessie:latest bash
+# % docker pull docker.mgm.sipwise.com/ngcpcfg-stretch
+# % docker run --rm -i -t -v $(pwd)/..:/code:rw docker.mgm.sipwise.com/ngcpcfg-stretch:latest bash
 #
 # Inside docker (the command is in history, just press UP button):
 #   ./t/testrunner