Dockerfile: pip installs using python2 -m

Change-Id: If7c44df62fd5fa37a0c1c0e289e73e13f5c6aa14
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/Dockerfile b/Dockerfile
index c30bf91..9c78473 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 pip==9.0.3 && \
-  DEBIAN_FRONTEND=noninteractive pip install -U pyang pyangbind && \
-  DEBIAN_FRONTEND=noninteractive pip install -U stdeb
+  DEBIAN_FRONTEND=noninteractive python2 -m pip install pip==9.0.3 && \
+  DEBIAN_FRONTEND=noninteractive python2 -m pip install -U pyang pyangbind && \
+  DEBIAN_FRONTEND=noninteractive python2 -m pip install -U stdeb