Change the priority of pip2 and pip3 installation

test=develop

Author: velconia

Dockerfile

@@ -72,34 +72,34 @@ RUN apt-get install -y python3 python3-dev python3-pip
 # specify sphinx version as 1.5.6 and remove -U option for [pip install -U
 # sphinx-rtd-theme] since -U option will cause sphinx being updated to newest
 # version(1.7.1 for now), which causes building documentation failed.
-RUN easy_install -U pip && \
+RUN pip3 install -U wheel && \
+    pip3 install -U docopt PyYAML sphinx==1.5.6 && \
+    pip3 install sphinx-rtd-theme==0.1.9 recommonmark
+    easy_install -U pip && \
     pip install -U wheel && \
     pip install -U docopt PyYAML sphinx==1.5.6 && \
     pip install sphinx-rtd-theme==0.1.9 recommonmark && \
-    pip3 install -U wheel && \
-    pip3 install -U docopt PyYAML sphinx==1.5.6 && \
-    pip3 install sphinx-rtd-theme==0.1.9 recommonmark
 
-RUN pip install pre-commit 'ipython==5.3.0' && \
-    pip install 'ipykernel==4.6.0' 'jupyter==1.0.0' && \
-    pip install opencv-python && \
-    pip3 install pre-commit 'ipython==5.3.0' && \
+RUN pip3 install pre-commit 'ipython==5.3.0' && \
     pip3 install 'ipykernel==4.6.0' 'jupyter==1.0.0' && \
     pip3 install opencv-python
+    pip install pre-commit 'ipython==5.3.0' && \
+    pip install 'ipykernel==4.6.0' 'jupyter==1.0.0' && \
+    pip install opencv-python && \
 
 #For docstring checker
-RUN pip install pylint pytest astroid isort LinkChecker
 RUN pip3 install pylint pytest astroid isort
+RUN pip install pylint pytest astroid isort LinkChecker
 
 COPY ./python/requirements.txt /root/
-RUN pip install -r /root/requirements.txt
 RUN pip3 install -r /root/requirements.txt
+RUN pip install -r /root/requirements.txt
 
 # To fix https://github.com/PaddlePaddle/Paddle/issues/1954, we use
 # the solution in https://urllib3.readthedocs.io/en/latest/user-guide.html#ssl-py2
 RUN apt-get install -y libssl-dev libffi-dev
-RUN pip install certifi urllib3[secure]
 RUN pip3 install certifi urllib3[secure]
+RUN pip install certifi urllib3[secure]
 
 
 # Install woboq_codebrowser to /woboq