Output image identity information

[colek42]

As a user I would like some way to create an attestation on the acorn image produced. There are two ways that I think we could do this.

1. Add a --iidfile flag that writes the image ID to a file. Docker currently has this flag

2. Add an option to output a tar image when running acorn build

[ibuildthecloud]

The —idfile is a simple addition that regardless of your use case we should do. But beyond that is there something we can do to make this even better?

[ibuildthecloud]

The tar option is a bit weird because acorn include multiple docker images so I’m not sure what the tar would be. Are you talking about a OCI formatted tar that has digests and tars in it? We can do that, but again it seems like we could do something cooler.