fix ModuleNotFoundError with a symlink

Author: gorogoroyasu

Dockerfile

@@ -46,4 +46,7 @@ RUN wget https://github.com/opencv/opencv/archive/${OPENCV_VERSION}.zip \
   .. \
 && make install \
 && rm /${OPENCV_VERSION}.zip \
-&& rm -r /opencv-${OPENCV_VERSION}
+&& rm -r /opencv-${OPENCV_VERSION} 
+RUN ln -s \
+  /usr/local/python/cv2/python-3.7/cv2.cpython-37m-x86_64-linux-gnu.so \
+  /usr/local/lib/python3.7/site-packages/cv2.so