c8d/inspect: Fix image inspect for incomplete images

[vvoland]

• fixes: Under certain conditions the API output of docker image inspect becomes corrupted #51566

When inspecting multi-platform images where some layer blobs were missing from the content store, the image inspect operation would return too early causing some data (like config details or unpacked size) to be omitted even though are available.

This ensures that docker image inspect returns as much information as possible.

- What I did

- How I did it

- How to verify it

TestImageInspect/inspect image with one layer missing unit test

- Human readable description for the release notes

containerd image store: Fix `docker image inspect` failing to return available image data in case where not all distributable blobs are available locally.

- A picture of a cute animal (not mandatory but encouraged)

[vvoland]

TODO, but perhaps still fine to merge if we were to do a patch release.

[vvoland]

A unit test is here:

moby/daemon/containerd/image_inspect_test.go

Lines 64 to 88 in 2e3a23c

	t.Run("inspect image with one layer missing", func(t *testing.T) {
	ctx := logtest.WithT(ctx, t)
	service := fakeImageService(t, ctx, cs)
	img := toContainerdImage(t, specialimage.MultiLayer)
	_, err := service.images.Create(ctx, img)
	assert.NilError(t, err)
	// Get the manifest to access the layers
	mfst, err := c8dimages.Manifest(ctx, cs, img.Target, nil)
	assert.NilError(t, err)
	assert.Check(t, len(mfst.Layers) > 0, "image should have at least one layer")
	// Delete the last layer from the content store
	lastLayer := mfst.Layers[len(mfst.Layers)-1]
	err = cs.Delete(ctx, lastLayer.Digest)
	assert.NilError(t, err)
	inspect, err := service.ImageInspect(ctx, img.Name, imagebackend.ImageInspectOpts{})
	assert.NilError(t, err)
	assert.Check(t, inspect.Config != nil)
	assert.Check(t, is.Len(inspect.RootFS.Layers, len(mfst.Layers)))
	})

[thaJeztah]

Could possibly move some of these together with where we construct the initial response. Did a quick try to see what it would look like; https://github.com/vvoland/moby/compare/c8d-fix-images...thaJeztah:51629_suggestions?expand=1

[vvoland]

We could, not sure if that's better though - currently there's a nicer separation around which fields come from config and it's also in the same place as the special handling for Comment and DiffIDs.

[thaJeztah]

LGTM