bugfix pip 10.0.0 issue in nettest dockerfile

Change-Id: Ie879efd662fba803cc5c6156c0981a0054e0f5da
Signed-off-by: zhihui wu <wu.zhihui1@zte.com.cn>

contrib/nettest/Dockerfile

@@ -33,7 +33,7 @@ RUN mkdir -p log
 VOLUME ["$DOCKYARD_SRCPROJ/log/"]
 
 # Install Python dependencies
-RUN pip install -U pip \
+RUN python -m pip install -U pip \
     && pip install -U setuptools \
     && pip install -r $DOCKYARD_SRCPROJ/requirements.txt