containerd integration: add container-count to image list (used for docker system df)

[thaJeztah]

Description

The existing implementation allows the number of containers using an image to be included in the result if opts.ContainerCount is set

moby/daemon/images/image_list.go

Lines 171 to 186 in b94f715

	if opts.ContainerCount {
	// Lazily init allContainers.
	if allContainers == nil {
	allContainers = i.containers.List()
	}
	// Get container count
	var containers int64
	for _, c := range allContainers {
	if c.ImageID == id {
	containers++
	}
	}
	// NOTE: By default, Containers is -1, or "not set"
	summary.Containers = containers
	}

⚠️ When using multiple snapshotters, what should be counted? As different containers may be using different snapshotters (and thus different snapshots for the same image)

[26tanishabanik]

@thaJeztah , can I work on this issue?

[thaJeztah]

Looks like some TODOs were left behind (not sure if all of them were done?) /cc @vvoland

moby/daemon/containerd/image_list.go

Lines 62 to 64 in 046827c

	// TODO(thaJeztah): implement opts.ContainerCount (used for docker system df); see https://github.com/moby/moby/issues/43853
	// TODO(thaJeztah): verify behavior of `RepoDigests` and `RepoTags` for images without (untagged) or multiple tags; see https://github.com/moby/moby/issues/43861
	// TODO(thaJeztah): verify "Size" vs "VirtualSize" in images; see https://github.com/moby/moby/issues/43862

[vvoland]

The TODO related to this issue is removed by: https://github.com/moby/moby/pull/47449/files#diff-849a807cb7508257e8a87abd4a72bb8e03937d568f7c74d48cc6dace908ce3d7L62