The existing docker push e2e test pushes a single 4KB scratch layer, too
small to exercise predastore's storage path for a real multi-hundred-MB
blob. Add a 3-layer (100+80+60MiB) push/pull case that verifies re-pulled
layer digests against the pushed image and directly checks Content-Length
on a raw HTTP GET of the largest blob, targeting a reported defect where
large blob GETs returned 200 with content-length: 0. Re-pull uses
`docker builder prune -af`, not just `rmi`, since buildx's build cache can
otherwise satisfy the pull without touching the registry.
Josh Siviour
a2b4b71840