feat: use containerd gc for image-fetcher cleanup

[awesomenix]

Summary

• use containerd's native GC label filtering for the image-fetcher pull path instead of scanning all images
• add an image-fetcher GC trigger entrypoint and invoke it once after the VHD preload batch completes
• enable discard of unpacked pulled layers during VHD image preloading while preserving fetch-only image blobs

Validation

• gofmt -w image-fetcher/main.go
• cd image-fetcher && go build ./...
• bash -n vhdbuilder/packer/install-dependencies.sh

[Copilot]

Pull request overview

This PR updates the VHD image preloading flow to rely on containerd’s native GC mechanics (via GC label filtering) rather than scanning images for cleanup, and adds an explicit GC trigger once the preload batch completes.

Changes:

• Add an --gc mode to image-fetcher that triggers containerd GC via lease create/delete.
• Update the image-fetcher pull path to apply containerd GC child-label filtering for layer blobs.
• Invoke image-fetcher --gc once after all parallel image pulls complete during VHD build.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
vhdbuilder/packer/install-dependencies.sh	Runs a single post-batch image-fetcher --gc after the image preload jobs finish.
image-fetcher/main.go	Adds --gc entrypoint and applies WithChildLabelMap(images.ChildGCLabelsFilterLayers) for the pull/unpack path.

[Copilot]

wait ${image_pids[@]} only returns the exit status of the last PID waited for, so failures in earlier background pulls can be missed (and with set -e the script would still proceed to run image-fetcher --gc). Consider iterating over PIDs (or looping wait -n until all jobs finish) and tracking any non-zero exit status so the build fails reliably before triggering GC.

Suggested change

	wait ${image_pids[@]}
	overall_status=0
	for pid in "${image_pids[@]}"; do
	if ! wait "$pid"; then
	status=$?
	echo "Container image pull job with PID $pid failed with exit code $status" >&2
	overall_status=$status
	fi
	done
	if [ "$overall_status" -ne 0 ]; then
	echo "One or more container image pulls failed; skipping image-fetcher GC and exiting with code $overall_status" >&2
	exit "$overall_status"
	fi