TT#17642 docker: follow policy to include NGCP repository

Change-Id: Iea4fdd07cef46bd113c9fafa9d8a47b87849a7a3

t/Dockerfile

@@ -5,13 +5,13 @@ 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 2017-05-08
+ENV REFRESHED_AT 2017-06-30
 
-# TODO - the release-trunk-jessie is too dynamic yet, though required build deps
-RUN echo "deb https://deb.sipwise.com/autobuild/ release-trunk-jessie main" >>/etc/apt/sources.list
+# 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 \
+RUN apt-get update && apt-get install --assume-yes \
         curl \
         nodejs
 
@@ -31,6 +31,12 @@ WORKDIR /code
 # ----------------------
 # When you want to build the base image from scratch
 # (jump to the next section if you don't want to build yourself!):
+#
+# you need to put the proper NGCP sources at t/sources.list.d/builddeps.list
+# for instance, trunk:
+# echo "deb https://deb.sipwise.com/autobuild/ release-trunk-jessie main" > /
+#  t/sources.list.d/builddeps.list
+#
 # NOTE: run the following command from root folder of git repository:
 # % docker build --tag="janus-client-jessie" -f ./t/Dockerfile .
 # NOTE: run the following command from root folder of git repository: