build-release: replace docker manifest create with docker buildx imagetools create

[Copilot]

docker manifest create fails when source tags are OCI manifest lists (produced by modern BuildKit), which is what per-arch docker build pushes generate. This caused the 1.8.0 release to fail with "docker.io/scylladb/vector-store:1.8.0-amd64 is a manifest list".

Changes

• scripts/build-release: Replace the final manifest-assembly step:
  • docker manifest create → docker buildx imagetools create -t "$docker_manifest" "${docker_arch_tags[@]}"
  • Add docker buildx imagetools inspect "$docker_manifest" to print and verify the resulting OCI image index in release logs

# Before
docker manifest create $docker_manifest "${docker_arch_tags[@]}"

# After
docker buildx imagetools create -t "$docker_manifest" "${docker_arch_tags[@]}"
docker buildx imagetools inspect "$docker_manifest"

imagetools create is designed to compose an OCI image index from existing manifest-list or single-manifest sources — the correct tool for this flow.

[swasik]

This will be addressed by #436