Use api3 as DOCKERHUB_USERNAME in docker-build CI workflow

[dcroote]

Closes #2034. Note that I didn't hardcode the variable everywhere- there are a a few publishing-related places below that retain it. The reason being that we've sometimes needed to push images to forks for testing and hardcoding api3 wouldn't be ideal, whereas hardcoding it in the CI workflow makes sense.

Other note: the CI docker build step is expected to fail because no changesets that affect packages have been added since the last release (see e.g. #1585 (comment))

airnode/package.json

Line 55 in 5b5e8c3

"docker:scripts:docker:publish": "docker run --rm -v /var/run/docker.sock:/var/run/docker.sock -e DOCKERHUB_USERNAME -e DOCKERHUB_TOKEN api3/airnode-packaging:latest docker publish",

airnode/docker/scripts/docker.ts

Lines 35 to 44 in 5b5e8c3

	const loginDockerHub = () => {
	const username = process.env.DOCKERHUB_USERNAME;
	const password = process.env.DOCKERHUB_TOKEN;
	if (!username || !password) {
	throw new Error('Missing DockerHub credentials');
	}
	runCommand(`docker login --password-stdin --username ${username}`, { input: password });
	};

and

airnode/docker/README.md

Lines 274 to 289 in 5b5e8c3

	You need to provide two environment variables to authenticate against the DockerHub registry:
	- `DOCKERHUB_USERNAME` - DockerHub username
	- `DOCKERHUB_TOKEN` - DockerHub access token
	Example:
	```bash
	docker run --rm -v /var/run/docker.sock:/var/run/docker.sock -e DOCKERHUB_USERNAME -e DOCKERHUB_TOKEN api3/airnode-packaging:latest docker publish
	```
	You can use a convenience Yarn target for publishing the latest Docker images:
	```bash
	docker:scripts:docker:publish:latest
	```

[Siegrift]

👍 LGTM