feat: manifest build with buildah allowing multi-platform images

Presentation

Closes #140 (closed)

Checklist

• Documented:
  • README.md documents the new feature
  • kicker.json describes the new feature
• Tested & examplified:
  • Build: https://gitlab.com/to-be-continuous/samples/python-on-kubernetes/-/jobs/11107394981
  • Pull: skopeo inspect docker://registry.gitlab.com/to-be-continuous/samples/python-on-kubernetes/snapshot:test-multi-arch --raw
  • Publish: https://gitlab.com/to-be-continuous/samples/python-on-kubernetes/-/jobs/11107832844

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.index.v1+json",
  "manifests": [
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:c72c92154241e0daf48419b64174e929535c925cf3e3ce6124d49ca1e77c0a46",
      "size": 2779,
      "platform": {
        "architecture": "amd64",
        "os": "linux"
      }
    },
    {
      "mediaType": "application/vnd.oci.image.manifest.v1+json",
      "digest": "sha256:ff4b8af000ac7b068970e670ba0d845a1c13aaae3b9d51fa4beb941e12a562b8",
      "size": 2780,
      "platform": {
        "architecture": "arm64",
        "os": "linux"
      }
    }
  ]
}

There are limit in the compatibility with trivy, syft and cosign. Nothing should fail but it is clearly suboptimal. See comment in the tracking issue.