add cmake to grpc_interop_cxx docker image

templates/tools/dockerfile/interoptest/grpc_interop_cxx/Dockerfile.template

@@ -19,6 +19,7 @@
   <%include file="../../apt_get_basic.include"/>
   <%include file="../../python_deps.include"/>
   <%include file="../../cxx_deps.include"/>
+  <%include file="../../cmake_jessie_backports.include"/>
   <%include file="../../run_tests_addons.include"/>
   # Define the default command.
   CMD ["bash"]

tools/dockerfile/interoptest/grpc_interop_cxx/Dockerfile

@@ -70,6 +70,15 @@ RUN pip install futures==2.2.0 enum34==1.0.4 protobuf==3.5.2.post1 six==1.10.0 t
 # C++ dependencies
 RUN apt-get update && apt-get -y install libgflags-dev libgtest-dev libc++-dev clang && apt-get clean
 
+#=================
+# Use cmake 3.6 from jessie-backports
+# should only be used for images based on debian jessie.
+
+RUN echo "deb http://archive.debian.org/debian jessie-backports main" | tee /etc/apt/sources.list.d/jessie-backports.list
+RUN echo 'Acquire::Check-Valid-Until "false";' > /etc/apt/apt.conf
+RUN sed -i '/deb http:\/\/deb.debian.org\/debian jessie-updates main/d' /etc/apt/sources.list
+RUN apt-get update && apt-get install -t jessie-backports -y cmake && apt-get clean
+
 
 RUN mkdir /var/local/jenkins