Created by: luotao1

After #11204, when run import cv2 in latest images:

root@f6bc27d7d69a:/# python
Python 2.7.12 (default, Dec  4 2017, 14:50:18)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import cv2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/cv2/__init__.py", line 3, in <module>
    from .cv2 import *
ImportError: libgthread-2.0.so.0: cannot open shared object file: No such file or directory
>>>

For missing libgthread-2.0.so.0, https://stackoverflow.com/questions/44205960/what-is-the-cause-of-this-strange-error-message-in-googles-cloud-datalab-when-i suggests to install libgtk2.0-dev, and our benchmark/fluid/Dockerfile also install libgtk2.0-dev. https://github.com/PaddlePaddle/Paddle/blob/4aaeeba73875ce96b62e206765037fd8d56258fe/benchmark/fluid/Dockerfile#L2