The Docker daemon emits a final `{"aux":{"Tag","Digest","Size"}}` JSON
object on the push event stream so clients can recover the registry-side
manifest digest without an extra registry round-trip.
Several Docker-API clients (e.g. the Pulumi and Terraform docker
providers) parse `aux.Digest` to populate their pushed-image outputs.
Without the trailer they either silently fall back to inspecting the
local image (whose digest does not always match what was pushed - see
containers/podman#14779) or emit "Push completed without reporting a
digest" warnings.
`pushReport.ManifestDigest` is already populated with the correct
on-the-wire digest from `manifest.Digest(pushedManifestBytes)`; this
change just wraps it into the same JSON shape Docker emits and adds an
apiv2 test asserting the trailer is present with Tag, Digest, and Size.
Signed-off-by: nitz <nitz.raz@gmail.com>