create-spec: fix for indexed images

[ktock]

#210

• fix platform to be correctly recognized
• convert only container manifest

[ktock]

confirmed images containing attestation manifest don't cause panic error anymore.

$ docker buildx build --attest type=sbom --output type=oci,dest=/tmp/sout2/img.tar /tmp/ctxs
$ mkdir /tmp/ex
$ tar -C /tmp/ex -xf /tmp/sout2/img.tar 
$ buildx build --progress=plain --build-arg TARGETARCH=amd64 --build-arg TARGETPLATFORM=linux/amd64 --platform=linux/amd64 -f Dockerfile --build-context assets=. --output type=local,dest=/tmp/out --build-arg OUTPUT_NAME=out.wasm --build-arg LINUX_LOGLEVEL=0 --build-arg INIT_DEBUG=false /tmp/soutex/
...
#29 0.065 Trying to unpack image as an OCI image
#29 0.066 nested manifest: processing [{application/vnd.oci.image.manifest.v1+json sha256:608ad19966c303f5bb8ed31d2e1de9aa1bb56a9b763b09d9eaddace01ec253f3 482 [] map[] [] 0xc000076480 }]