question: crane mutate/append with local tarball as a base image

Hi,

I'm trying to append or mutate a local tar:
```sh
crane mutate node.tar \
--append "$tar" \
--output "$image" \
--tag "$project" \
--cmd "./src/index.js"
```

And i'm getting the following error:
```sh
Error: pulling node.tar: parsing reference "node.tar": could not parse reference: node.tar
```

I want to use a local tar since am pulling the base image beforehand in order to cache the layers (it's not possible to cache with crane mutate/append):
```sh
crane pull "$base" "node.tar" --cache_path cache
```

Start with the crane mutate and append command paths, then compare them with the crane pull invocation shown in the report. Check how image references are parsed and whether local tar inputs are supported; done means the project has a clear resolution for using node.tar as the base.

Written by the indexing model from the issue text.