cannot import OpenCV in a Docker container in Raspberry Pi
Seems to be just an issue with a Python package.

Traceback (most recent call last):
  File "a.py", line 1, in &lt;module&gt;
    import cv2
ImportError: No module named 'cv2'


Is a common Python error, basically it can't find the package cv2.

You could try and install OpenCV through pip instead of installing through apt get.
Something could be up with dependancies that you may have missed.

RUN pip3 install opencv-python