Add tests for c-ares wrapper using a local DNS server.
diff --git a/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile b/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile
index 349a6ef..61f005d 100644
--- a/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile
+++ b/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile
@@ -22,6 +22,7 @@
   bzip2 \
   ccache \
   curl \
+  dnsutils \
   gcc \
   gcc-multilib \
   git \
@@ -65,7 +66,7 @@
 # Install Python packages from PyPI
 RUN pip install pip --upgrade
 RUN pip install virtualenv
-RUN pip install futures==2.2.0 enum34==1.0.4 protobuf==3.2.0 six==1.10.0
+RUN pip install futures==2.2.0 enum34==1.0.4 protobuf==3.2.0 six==1.10.0 twisted==17.5.0
 
 #=================
 # C++ dependencies