Cache slow/computed keys using an LRU #256

Job	Run time
prepare	1s
test / prepare	2m 10s
test / run (containerd, ./frontend/dockerfile, integration)	3m 20s
test / run (containerd, ./frontend/dockerfile, dockerfile)	3m 39s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	3m 52s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	4m 1s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	3m 23s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	3m 55s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 9s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 12s
test / run (oci, ./frontend/dockerfile, integration)	3m 41s
test / run (oci, ./frontend/dockerfile, dockerfile)	4m 46s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 38s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 42s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 9s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 41s
image	0s
release	0s
	57m 19s

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