We should standardize some of the interactions with composefs and OCI. Today the composefs tooling is very generic, and integration with OCI or other ecosystems is left to do externally (as is happening in e.g. containers/storage).

Embedding containers.composefs-digest as metadata

While this is a broad topic the first example I'd give here is that we should standardize embedding the composefs digest in a container image manifest; much as was done with ostree and embedding it in in the commit metadata.

Something like a standard containers.composefs-digest (bikeshed: label or annotation?). And we should define exactly how a container image is mapped to a composefs tree. Specifically, I would argue here that the embedded digest should be of the merged, flattened filesystem tree - and that's actually how it should be mounted as well (instead of doing it via individual overlayfs mounts) - i.e. we'd do it how ostree does it.

However, it wouldn't hurt to also embed an annotation with the composefs digest for each individual layer (as part of the descriptor metadata) to give a runtime the ability to selectively choose to manage individual layers or not.

Finally of course, it would make sense for us to provide some tooling which does this. It's an interesting question, should there be something like podman build --feature=composefs to auto-inject this? But in the general case we can just provide a simple tool that accepts an arbitrary container image and "re-processes" it to add this metadata.