Update Dockerfile

Dockerfile

 FROM yoanlin/opencv-python3:latest
 RUN apt-get update
 RUN pip install --upgrade pip
-RUN pip install --upgrade tensorflow
+RUN pip install tensorflow==2.0.0-beta1 
 RUN pip install numpy scipy scikit-learn pillow h5py keras 
 RUN pip install --upgrade imutils 
 RUN pip install --upgrade scikit-learn