Accept any python version in Dockerfile

[zstadler]

Avoid the hard-coded python3.7 version and this error:

Step 4/4 : RUN export GEOS_VERSION="$(python setup.py --version)"     && ln -s /opt/conda/lib/python3.7/site-packages/geos-"$GEOS_VERSION"-py3.7.egg/geos /opt/conda/lib/python3.7/site-packages/geos
 ---> Running in 5a9c9d929409
ln: failed to create symbolic link '/opt/conda/lib/python3.7/site-packages/geos': No such file or directory
The command '/bin/sh -c export GEOS_VERSION="$(python setup.py --version)"     && ln -s /opt/conda/lib/python3.7/site-packages/geos-"$GEOS_VERSION"-py3.7.egg/geos /opt/conda/lib/python3.7/site-packages/geos' returned a non-zero code: 1

[grst]

Thanks again!