Update FAQ.md, elaborate on how to customise docker image and pick 'python:3' to solve typical errors (#269)

* Update FAQ.md

* Update website/docs/FAQ.md

Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>

* Update website/docs/FAQ.md

Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>

* Update website/docs/FAQ.md

Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>

---------

Co-authored-by: Qingyun Wu <qingyun.wu@psu.edu>