Add local src to PYTHONPATH

[RagingTiger]

Should be something like: ENV PYTHONPATH=${PYTHONPATH}:/home/jovyan/src

References:

• https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH
• https://bic-berkeley.github.io/psych-214-fall-2016/using_pythonpath.html

[RagingTiger]

Updated Makefile in https://github.com/RagingTiger/cookiecutter-data-science/commit/1964e0bae588e908dea736daa67a9d3e5e60c7cf to set PYTHONPATH on docker run command (i.e. make jupyter).