In general I believe it's clever to synthesize an index for
single-platform image manifest, but we still need a way to distinguish
it. Add a dedicated annotation is the slightest change I've come up
with, and it's also OCI compliant. With this change come in, we can work
around https://github.com/apple/container/issues/212 and imitate the
behavior of other runtime with `container`.
Note that since `cctl` is meant to be a dedicated tool for inspecting
the Containerization framework itself, I didn't apply the indirection
for it, and it will be as-is with the genuine storage.