Bake build limited to single target

We currently limit Bake build to a single target: https://github.com/docker/github-builder-experimental/blob/e875129c872af249a9411fc0b1b1d1fe79ee6ddf/.github/workflows/bake.yml#L226-L228

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

リンク先の .github/workflows/bake.yml の 226-228 行目から始めて、単一ターゲットの制限について理解し、その後、リンク先の bake-action subaction/matrix の例を確認してください。Bake が 1 つのターゲットに制限されている理由を文書化し、ユーザーがマトリックス戦略を使用して複数のターゲットに対応する方法を説明してください。