Docker: universal (arm64/amd64) builds

[peterwilli]

Intro

I added the platforms tag to create both ARM and x86 builds. This lets us use the bootcamp on devices such as the M series Macs.

While I'm aware there's a mybinder link that lets you run the bootcamp in the browser, it often goes offline and you lose your process.

Testing that was done

1. Tested the new workflow with Act (https://github.com/nektos/act, a tool to locally run GitHub workflows), it still builds
2. Pushed the Docker image under my own repo and ran it on my m2 Macbook Air, I can run the bootcamp in the notebooks!

It should be a drop-in replacement with no further action required