ostree container image deploy: support platform variant

Some old x86_64 devices require variant "v2" to be set explicitly with images defaulting to "v3".

I make such images with:

bluebuild build -vv -S sigstore --platform linux/amd64/v2 --push ./recipes/$RECIPE

Unfortunately, this breaks ostree:


This is the skopeo output:

~~~json
skopeo inspect --raw docker://registry.gitlab.com/eu-os/workspace-images/eu-os-base-demo/eu-os-demo:latest
{
"schemaVersion": 2,
"mediaType": "application/vnd.oci.image.index.v1+json",
"manifests": [
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"digest": "sha256:45b6d9b0aeb1eb149dae0b6d6d3a446ea20d8a47d67df6aa6fc7cbca29d97f75",
"size": 22226,
"platform": {
"architecture": "amd64",
"os": "linux",
"variant": "v2"
}
},
{
"mediaType": "application/vnd.oci.image.manifest.v1+json",
"digest": "sha256:ce197c50c59a449e9ad11f7111ac92db84fdeca190138909b7ff334546a00334",
"size": 566,
"annotations": {
"vnd.docker.reference.digest": "sha256:45b6d9b0aeb1eb149dae0b6d6d3a446ea20d8a47d67df6aa6fc7cbca29d97f75",
"vnd.docker.reference.type": "attestation-manifest"
},
"platform": {
"architecture": "unknown",
"os": "unknown"
}
}
]
}⏎
~~~

Can we please add a way to configure variant and document it for both ostree and bootc?