Restore safe Docker publishing

[colincasey]

This fix restores the safety of triggering the release automation on an existing release which was lost in the updates for publishing multi-arch Docker images in #194.

The buildpack matrix output can contain two variants of images to publish:

• A CNB with a single target. E.g.;

  {
    "buildpack_id": "heroku/nodejs-function",
    "buildpack_version": "3.2.6",
    "buildpack_type": "composite",
    "buildpack_dir": "/home/runner/work/buildpacks-nodejs/buildpacks-nodejs/meta-buildpacks/nodejs-function",
    "targets": [
      {
        "os": "linux",
        "arch": "amd64",
        "rust_triple": "x86_64-unknown-linux-musl",
        "oci_target": "linux/amd64",
        "cnb_file": "heroku_nodejs-function.cnb",
        "stable_tag": "docker.io/heroku/buildpack-nodejs-function:3.2.6",
        "temporary_tag": "docker.io/heroku/buildpack-nodejs-function:_9781380433",
        "output_dir": "/home/runner/work/buildpacks-nodejs/buildpacks-nodejs/./packaged/x86_64-unknown-linux-musl/release/heroku_nodejs-function"
      }
    ],
    "image_repository": "docker.io/heroku/buildpack-nodejs-function",
    "stable_tag": "docker.io/heroku/buildpack-nodejs-function:3.2.6",
    "temporary_tag": "docker.io/heroku/buildpack-nodejs-function:_9781380433"
  }

• Or a CNB with multiple targets. E.g.;

  {
    "buildpack_id": "heroku/nodejs-npm-install",
    "buildpack_version": "3.2.6",
    "buildpack_type": "libcnb",
    "buildpack_dir": "/home/runner/work/buildpacks-nodejs/buildpacks-nodejs/buildpacks/nodejs-npm-install",
    "targets": [
      {
        "os": "linux",
        "arch": "amd64",
        "rust_triple": "x86_64-unknown-linux-musl",
        "oci_target": "linux/amd64",
        "cnb_file": "heroku_nodejs-npm-install_linux-amd64.cnb",
        "stable_tag": "docker.io/heroku/buildpack-nodejs-npm-install:3.2.6_linux-amd64",
        "temporary_tag": "docker.io/heroku/buildpack-nodejs-npm-install:_9781380433_linux-amd64",
        "output_dir": "/home/runner/work/buildpacks-nodejs/buildpacks-nodejs/./packaged/x86_64-unknown-linux-musl/release/heroku_nodejs-npm-install"
      },
      {
        "os": "linux",
        "arch": "arm64",
        "rust_triple": "aarch64-unknown-linux-musl",
        "oci_target": "linux/arm64",
        "cnb_file": "heroku_nodejs-npm-install_linux-arm64.cnb",
        "stable_tag": "docker.io/heroku/buildpack-nodejs-npm-install:3.2.6_linux-arm64",
        "temporary_tag": "docker.io/heroku/buildpack-nodejs-npm-install:_9781380433_linux-arm64",
        "output_dir": "/home/runner/work/buildpacks-nodejs/buildpacks-nodejs/./packaged/aarch64-unknown-linux-musl/release/heroku_nodejs-npm-install"
      }
    ],
    "image_repository": "docker.io/heroku/buildpack-nodejs-npm-install",
    "stable_tag": "docker.io/heroku/buildpack-nodejs-npm-install:3.2.6",
    "temporary_tag": "docker.io/heroku/buildpack-nodejs-npm-install:_9781380433"
  }

When the automation runs successfully, either a single image or multiple images + a manifest list will have been published. Both the single image and manifest list are tagged with the value matrix.stable_tag. Therefore, if this tag exists, we shouldn't need to publish anything to Docker and we can skip that work.

Fixes #238

[edmorley]

Thank you for fixing this! :-)

[colincasey]

Verified it's working as expected in this test run:
https://github.com/heroku/buildpacks-nodejs/actions/runs/9799098390/job/27058768319