Fix Dockerfile to install pip9.0.3 instead of latest pip

Change-Id: Id3da61fb9c6bb2f93daedd6c54598aa8bb649e15
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/Dockerfile b/Dockerfile
index 1e94793..c30bf91 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -18,7 +18,7 @@
 
 RUN  apt-get update && \
   DEBIAN_FRONTEND=noninteractive apt-get -y install git make wget python python-pip debhelper tox && \
-  DEBIAN_FRONTEND=noninteractive pip install -U pip && \
+  DEBIAN_FRONTEND=noninteractive pip install pip==9.0.3 && \
   DEBIAN_FRONTEND=noninteractive pip install -U pyang pyangbind && \
   DEBIAN_FRONTEND=noninteractive pip install -U stdeb