Verify ignore scratch images

[rjbrown57]

Fixes #508

[dekkagaijin]

Is there a use-case where "scratch" is being verified directly with verify? Or is meant to fix an issue with verify-dockerfile? If the latter, it'd be better to patch that command. FWIW I thought I had already special-cased FROM scratch in verify-dockerfile but I was clearly mistaken

[rjbrown57]

Is there a use-case where "scratch" is being verified directly with verify? Or is meant to fix an issue with verify-dockerfile? If the latter, it'd be better to patch that command. FWIW I thought I had already special-cased FROM scratch in verify-dockerfile but I was clearly mistaken

At least when a user is running with docker and "scratch" defaults to "https://index.docker.io/v2/library/scratch" I don't think anyone will run verify scratch.

Would it make more sense to filter somewhere in https://github.com/sigstore/cosign/blob/main/cmd/cosign/cli/verify_dockerfile.go#L115?

[rjbrown57]

Updated to ignore scratch in verify-dockerfile