This is a simple PR that just adds two options to the Makefile for the Docker image.

The gist is that I prefer jupyter lab over jupyter notebook (and it's now the "standard" interface anyway). So this just gives a convenient way to invoke the lab version in docker instead of the notebook version.

I know that it's not that hard to get to lab from a "notebook" server anyway (and vice versa) but I prefer lab because it's habit at this point and the URLs are slightly different so not all tooling is cross-compatible.

(There's also a trivial comment typo that I rolled in here as a second commit).