ETA Dockerfile does not support visualiziation

[brimoor]

Running the examples from within the ETA Docker image:

cd /path/to/eta/examples/
python download_data.py
python demo_image_manipulation/demo_images.py

yields the following error:

Uncaught exception
Traceback (most recent call last):
  File "demo_image_manipulation/demo_images.py", line 46, in <module>
    plot(img3)
  File "demo_image_manipulation/demo_images.py", line 30, in plot
    cv2.imshow("*** Press any key to exit ***", etai.rgb_to_bgr(img))
cv2.error: OpenCV(4.1.0) /io/opencv/modules/highgui/src/window.cpp:627: error: (-2:Unspecified error) The function is not implemented. Rebuild the library with Windows, GTK+ 2.x or Cocoa support. If you are on Ubuntu or Debian, install libgtk2.0-dev and pkg-config, then re-run cmake or configure script in function 'cvShowImage'

The Dockerfile needs more packages, it seems.