unpack: useless config.json

[cyphar]

The config.json generated when unpacking is not useful because it is a config for running in the host, which isn't helpful. While this can be corrected by a user (using oci-runtime-tools generate) this quite a bit of a pain.

The best choice IMO would be to create a library that can take a v1.Image and then apply it to a runtime-tools/generate.Generator. We can then use the default config for runC as the base (which we know is actually sane).

The upstream bug for this is opencontainers/image-tools#76 but we should implement this ourselves.

[cyphar]

This has been fixed in 3c4ef3f and 43efb56. Though we need to re-implement everything locally (this is just a hack).