ci(publish): add key for matrix.target

[fzipi]

what

• add key to index proper container image metadata

why

• fix release

tests

Using this sampled json output, from one of the failed builds (notice the added buildx.build.warnings):

{
  "apache-alpine-3-3-7": {
    "buildx.build.ref": "builder-b9bbe4fa-025e-4bb2-8c9a-8d657e3e0312/builder-b9bbe4fa-025e-4bb2-8c9a-8d657e3e03120/325v4oju7bmxtxjrfxkl584vd",
    "containerimage.descriptor": {
      "mediaType": "application/vnd.oci.image.index.v1+json",
      "digest": "sha256:90884c176e54d5333a1b8ce11d07b77c5742ac9df0ede39af737c6cee2608306",
      "size": 3136
    },
    "containerimage.digest": "sha256:90884c176e54d5333a1b8ce11d07b77c5742ac9df0ede39af737c6cee2608306",
    "image.name": "owasp"
  },
  "buildx.build.warnings": [
    {
      "vertex": "1"
    }
  ]
}

We get:

cat sample.json| jq -r '."apache-alpine-3-3-7" ."containerimage.digest"'
sha256:90884c176e54d5333a1b8ce11d07b77c5742ac9df0ede39af737c6cee2608306

and

cat sample.json| jq -r '."apache-alpine-3-3-7" ."image.name"'
owasp