Cache slow/computed keys using an LRU #255

Job	Run time
prepare	2s
test / prepare	5m 21s
test / run (containerd, ./frontend/dockerfile, integration)	3m 13s
test / run (containerd, ./frontend/dockerfile, dockerfile)	3m 53s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	3m 21s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	4m 20s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	3m 20s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	3m 51s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 46s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 31s
test / run (oci, ./frontend/dockerfile, integration)	3m 40s
test / run (oci, ./frontend/dockerfile, dockerfile)	4m 15s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 25s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	4m 12s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 6s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 32s
image	0s
release	0s
	56m 48s

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