Proposal: Extend the TaskRun Status to contain the exact digest of all images used as steps.

[dlorenc]

Expected Behavior

It should be possible to determine what images (including digest) were used in a TaskRun after it is complete.

This information is available on a Pod right now under Status.ContainerStatus.ImageID. We could obtain this later by finding the Pod used for a TaskRun and getting the digests there, but we should expect that completed pods will eventually be GC'ed. The digests are likely something we want to ensure remains available later.

Actual Behavior

It is not possible to determine the digests of images used in a TaskRun today.

cc @imjasonh @bobcatfish