Support emitting a fully-qualified digest

[mattmoor]

When mink-image-target is specified, we wire up the --image functionality here:

mink/pkg/command/run.go

Line 218 in fc126e6

if paramNames.Has(constants.ImageTargetParam) {

We also have a special result name for mink-image-digest, which holds the sha256:deadbeef portion of the digest, but doesn't yet light up any additional functionality.

This issue is to track adding a PostRunFunc that emits {image}@{digest} by default when both of these special names are specified in their respective places.

[mattmoor]

this is done