opencv-python support TPU VM

Fixing a bug where import cv2 on TPU VM wouldn't work.

Author: djherbis

tpu/Dockerfile

@@ -38,6 +38,11 @@ ADD patches/kaggle_datasets.py /root/.local/lib/${PYTHON_VERSION_PATH}/site-pack
 # sitecustomize adds significant latency to ipython kernel startup and should only be added if needed
 # ADD patches/sitecustomize.py /root/.local/lib/${PYTHON_VERSION_PATH}/site-packages/sitecustomize.py
 
+# Prereqs
+# This is needed for cv2 (opencv-python):
+# https://stackoverflow.com/questions/55313610/importerror-libgl-so-1-cannot-open-shared-object-file-no-such-file-or-directo
+RUN apt-get update && apt-get install ffmpeg libsm6 libxext6  -y
+
 # Install all the packages together for maximum compatibility.
 
 # Install Tensorflow.