Cache slow/computed keys using an LRU #259

Job	Run time
prepare	1s
test / prepare	1m 38s
test / run (containerd, ./frontend/dockerfile, integration)	5m 57s
test / run (containerd, ./frontend/dockerfile, dockerfile)	3m 23s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 13s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	4m 0s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	3m 26s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	3m 18s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 30s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 55s
test / run (oci, ./frontend/dockerfile, integration)	3m 17s
test / run (oci, ./frontend/dockerfile, dockerfile)	3m 50s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 12s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 31s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 46s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 0s
image	0s
release	0s
	54m 57s

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