title: cv2.VideoCapture doesn't work within docker container
body: <p>I am trying to use cv2.VideoCapture to capture image from a docker container.</p>
  
  <p><code>import cv2
  vid = cv2.VideoCapture('path\to\video')
  ret, frame = vid.read()</code></p>
  
  <p>In terms of the video file,
  I have tried 
  either mount the file with <code>docker -v</code>
  or <code>docker cp</code> to copy the video file into container,
  but both with no luck (ret returns False).</p>
  
  <p>Should I add any command when launching the container?
  Thanks in advance.</p>