The integration tests started failing.
I don't know why. I don't understand docker and trying to figure it out was really opaque.
The new version of docker desktop does have a UI - but it is pretty useless unless you PAY for docker...

I tried upgrading the docker image to the node16 image (alpine), but that introduced a different error saying that the shell script was missing - even though my `ls` showed the file was in the exact place on the container's disk. I spent about an hour trying to debug this - it was all super confusing because I've never properly used docker before.
At this point - I'm kind of done with docker for this.
It seems like overkill anyway considering all we need is a folder isolated from our local `node_modules` folder.

This PR replaces the docker testing infra with a new pure JS version.
It's a like-for-like change, with the following differences:

- Instead of using a container to isolate the test - it creates a temporary folder.
- Instead of packaging and installing our packages locally once per container - it does it once for all tests.
- Instead of executing shell scripts - it executes commands (`yarn install` and `eslint` via node).
- It uses `yarn` instead of `npm` so that installs can use an offline cache (so they are faster).
- Instead of separately executing jest, it asserts the snapshot as part of the script.
- Each test is a separate jest test file, which makes it easy to snapshot and execute the test.

Benefits:
- Much faster than the docker solution - each individual test takes around 9-12s locally.
- Less opaque for other contributors as there is no docker magic

Downsides:
- Not *truly* isolated as it can still be influenced by the local dev environment - I don't think this is a huge deal for ESLint.