Bake build limited to single target

[crazy-max]

We currently limit Bake build to a single target:

github-builder-experimental/.github/workflows/bake.yml

Lines 226 to 228 in e875129

	if (targets.length > 1) {
	throw new Error(`Only one target can be built at once, found: ${targets.join(', ')}`);
	}

Mainly because we don't have a way to discern which generated tags are associated to a target but also because a target could output build result locally while another one would push to registry.

We should document this but also explain how users can use a matrix strategy to solve this issue. For example using our matrix subaction: https://github.com/docker/bake-action/tree/master/subaction/matrix