Add plotly to the docker image

[trevorcampbell]

I think plotly may be missing. The pre-rendered images from when the MDS students were doing their pass are still committed though, and they're fine, so no need to do this right now.

[joelostblom]

Yup I can confirm that it is missing. Running docker run --rm -v $(pwd):/home/jovyan ubcdsci/py-intro-to-ds:20230104230634037f38 /bin/bash -c "python -c 'import plotly'" returns ModuleNotFoundError: No module named 'plotly'. Fixing this should fix #83 as well.