Add Dockerfile

[peanutbutterandcrackers]

Hey there,

This change adds a Dockerfile (and also a Makefile, and documents their usage in README.md).

This is my 4th PR to this project, and yet I don't want to install Flask on my machine. Maybe there are more people like myself out there who are just as picky. This will help them to contribute to this project. Their workflow could be something like:

1. Make Changes
2. make [byol-docker]
3. Test the locally hosted website at localhost:5000 (by default).
4. Repeat till good-enough.

Hope this helps.

Thank you for the awesome book!