Remove or explain commented-out code in Dockerfile

[martin-martin]

There are unused imports in the example Dockerfile in week02/labs/README.md:

# FROM nvidia/cuda
# FROM nvidia/cuda:8.0-cudnn6-devel
# FROM nvidia/cuda:8.0-cudnn5-devel
--snip--
#  CMD jupyter notebook --no-browser --ip=0.0.0.0 --NotebookApp.token= --allow-root

These are unexplained and seem to be unused and can be confusing for students.

I suggest to remove the commented-out lines, or explain their raison d'être.