Add devcontainer.json

[lancechua]

Hi,

Thanks a ton for this course!

I've added a devcontainer.json spec to make it easier for other students to get set up. The key thing is to use Ubuntu 20.04 since some code unfortunately doesn't work with later versions of glibc.

This spec is also supported by more open source projects like devpod, which can also use AWS as a provider, though I think even just the Dockerfile will already be helpful for those not inclined to use codespaces.