Make Python installation scriptable through Docker build-args in Makefile (!5)

By setting Docker --build-args in the Makefile which are passed to the docker build and then to install_python.sh as command line parameters, the Python version tag number and if python and pip should be symlinked to python3 and pip3 can be controlled for different builds.

* Add Docker build-args to the Makefile to control the Python version number and if python and pip should be symlinked to python3 and pip3
* Add the build-args to all Docker images