How to display images/movies from Docker

After building Opnecv with the Docker container up,

import numpy as np
import cv2

cap=cv2.VideoCapture('video_name.mp4')
while(cap.isOpened()):

 ret, frame=cap.read()

      gray=cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY)

      cv2.imshow('frame',gray)
      if cv2.waitKey(1)&0xFF==ord('q'):
          break

cap.release()
cv2.destroyAllWindows()

and
(frame:309):Gtk-WARNING**:cannot open display:
Error similar to

I put ffmpeg in when building with opencv, so I think I can load the video, but how do I display the images and videos from docker?