Prioritize earlier layers during download

[omerXfaruq]

Problem

Within one image, layers compete for the same shared concurrency budget with no priority. If layer 1 and layer 2 are racing for bytes, they split bandwidth evenly, but only layer 1's completion is on the unpack critical path (overlay snapshots commit sequentially by chain id). Prioritizing earlier layers will speed up the unpack critical path, which is the bottleneck most of the time.

Code references

• Layers dispatched as siblings, no ordering. images.Dispatch launches one eg.Go per descriptor; the semaphore caps how many run, but scheduling order is whatever the Go runtime picks manifest index is not consulted.

  containerd/core/images/handlers.go

  Line 156 in 2976f38

  func Dispatch(ctx context.Context, handler Handler, limiter *semaphore.Weighted, descs ...ocispec.Descriptor) error {

• Same r.Acquire(ctx, 1) pool guards all in-flight requests across layers.

  containerd/core/remotes/docker/fetcher.go

  Line 461 in 2976f38

  if err := r.Acquire(ctx, 1); err != nil {

• Unpack is sequential by chain id, even in parallel mode.

  containerd/core/unpack/unpacker.go

  Line 369 in 2976f38

  // pre-calculate chain ids for each layer

Happy to open up a PR or discuss planned approach if there is interest. cc @samuelkarp @dmcgowan