Add Docker build

[TJC]

This PR adds support for building the code into a Docker container, and then run it from within that.

I note that in the process, I had to kind of hack index.js in two ways, which you might want to fix before you merge it into your main branch.

• I added --no-sandbox to the Chrome args, since that is required to run it as root. (And it runs as root inside the Docker container)
• I changed the path to Chromium to match the one used in the container

There are probably better ways of achieving those - I suggested using an ENV var for the path, which I imagine is easy for you to fix as you're familiar with Nodejs. See comment in the code.

[lukealford]

This will need more adjustments before it gets merged, hard setting the puppeteer env will break all other systems period. I'll look at this in the near future.

[TJC]

Hi Luke -- I've made some improvements to the script now.