[OCI wasm images] Look into using different field for looking up WASM layers

1. This doesn't work with Docker. Not your fault since Docker is not filling in `container.Image` which is how you are traversing the content. I have a patch prepared (for docker) to remedy this, but it would only work when docker is using containerd-backed storage.
2. `container.Image` is not the best data to use... we should add a new field to the containerd protos (in containerd/containerd) for an immutable reference to the image manifest being used... for now we'd need to work with what we've got

We may be able to get an immutable ref to the image by going through the container's snapshot.

We can also modify containerd but we should wait until we solidify on the WASM OCI artifact (there is a WG in the CNCF wg-wasm sorting this out now)