[Bug] content/index.json has empty manifest list

[sparr]

Description

/var/lib/soci-snapshotter-grpc/content/index.json contains "manifests":[] where it seems there should be a list of the manifests present in the content store.

Steps to reproduce the bug

1. soci create for any image that produces an index and ztocs
2. cat /var/lib/soci-snapshotter-grpc/content/index.json

Describe the results you expected

the newly created index manifest listed in index.json

Host information

N/A

Any additional context or information about the bug

WARNING
Resolving this situation will slightly increase the startup time of soci and soci-snapshotter-grpc while they read in each existing manifest.

Resolving this situation would allow our existing use of oci.New() to successfully loadIndexFile and then graph.IndexAll. Note that the "index" in "index.json" and "IndexFile" relates to the subject of this issue involving a list of manifests, while the "index" in "IndexAll" is a separate concept referring to an internal mapping between manifests and zTOCs.

https://github.com/opencontainers/image-spec/blob/main/image-layout.md#indexjson-file describes the expected contents of this file.

https://pkg.go.dev/oras.land/oras-go/pkg/content#OCI.SaveIndex is the method to export this file after changes have been made to the Store.index