treecompose: properly record sha256sum

[miabbott]

Using podman inspect to get the Id of a container locally, is not
the same as getting the Digest from the registry. This changes the
pipeline to use the value of Digest from the registry.

[miabbott]

For example:

$ sudo podman inspect --format='{{.Id}}' registry.svc.ci.openshift.org/rhcos/os-maipo:latest
eb74aca9891a87b35395850df04eb01c17e30237c3b8b95d1f0d13ffbb23147a

$ sudo podman pull registry.svc.ci.openshift.org/rhcos/os-maipo@sha256:eb74aca9891a87b35395850df04eb01c17e30237c3b8b95d1f0d13ffbb23147a
Trying to pull registry.svc.ci.openshift.org/rhcos/os-maipo@sha256:eb74aca9891a87b35395850df04eb01c17e30237c3b8b95d1f0d13ffbb23147a...Failed
error pulling image "registry.svc.ci.openshift.org/rhcos/os-maipo@sha256:eb74aca9891a87b35395850df04eb01c17e30237c3b8b95d1f0d13ffbb23147a": unable to pull registry.svc.ci.openshift.org/rhcos/os-maipo@sha256:eb7s

$ sudo skopeo inspect docker://registry.svc.ci.openshift.org/rhcos/os-maipo:latest | sudo podman run --rm -i docker.io/miabbott/aht-tools jq '.Digest'
"sha256:513731b35f927bf3c954dec3074bdcf2c669150948598e717469584cca21fcb9"

$ sudo podman pull registry.svc.ci.openshift.org/rhcos/os-maipo@sha256:513731b35f927bf3c954dec3074bdcf2c669150948598e717469584cca21fcb9
Trying to pull registry.svc.ci.openshift.org/rhcos/os-maipo@sha256:513731b35f927bf3c954dec3074bdcf2c669150948598e717469584cca21fcb9...Getting image source signatures
Skipping fetch of repeat blob sha256:8aa098d03c3bc08d86d97a2c5e0a116953ff2ef7353413ecaa357485452cff72
Skipping fetch of repeat blob sha256:132fa26b77c6df2e77877b1f1786e863760f97093717b3b35be68c1d255680a2
Copying config sha256:eb74aca9891a87b35395850df04eb01c17e30237c3b8b95d1f0d13ffbb23147a
 823 B / 823 B [============================================================] 0s
Writing manifest to image destination
Storing signatures
eb74aca9891a87b35395850df04eb01c17e30237c3b8b95d1f0d13ffbb23147a

[cgwalters]

/lgtm