fix simbolik link to cv python shared library

[epifanio]

the simbolic link to the cv python library should point to the installed python version, so if you use python3.9 it should be:

    RUN ln -s \
    /usr/local/python/cv2/python-3.9/cv2.cpython-39m-x86_64-linux-gnu.so \
    /usr/local/lib/python3.8/site-packages/cv2.so

However, this hard fix is no longer needed (at least with recent cmake and latest opencv version)