containerd: add option to set parent cgroup #3817

Job	Run time
build (windows/amd64)	3m 4s
build (freebsd/amd64)	2m 42s
test-freebsd-amd64	24m 12s
test-windows-amd64 (containerd, ./client)	4m 51s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 54s
test-windows-amd64 (containerd, ./worker/containerd)	3m 14s
test-windows-amd64 (containerd, ./solver)	3m 41s
test-windows-amd64 (containerd, ./frontend)	3m 11s
test-windows-amd64 (containerd, ./frontend/dockerfile)	5m 59s
test-windows-amd64 (./..., 1)	14m 11s
	1h 7m 59s

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