Migrate to GitHub Actions

[foolip]

Part of whatwg/meta#173.

[foolip]

I'm done tinkering with this now, but can't test that it's really going to work without publishing something to Docker Hub. I'd say it's 80% likely to work, so we could land it and see what happens.

[domenic]

Let's try it!

[domenic]

It worked! https://hub.docker.com/repository/docker/whatwg/wattsi/tags?page=1

I wonder if using the official Docker action would have let us avoid the warnings on https://github.com/whatwg/wattsi/runs/777797241?check_suite_focus=true#step:6:9

[domenic]

Looks like no: docker/build-push-action#53 and https://github.com/docker/github-actions/blob/0f18e2abad9a4ac2963d2516246787375b5ec917/internal/command/args.go#L9

[foolip]

We could easily use --password-stdin and silence the warning, but we wouldn't really be improving the security, I think. Anyone with write access to this repo would be able to extract the secrets if they want to. The other attack vector would be some software running in the agent, but we're trusting GitHub with our secrets and to vet the software, so...

[domenic]

Yeah, I'm happy to wait for whatever the Docker team comes up with, and either copy it or revisit whether we can use the Docker action.