Cache IDs & fix BK result caching #170

Job	Run time
prepare	1s
test / prepare	7m 54s
test / run (containerd, ./frontend/dockerfile, integration)	3m 32s
test / run (containerd, ./frontend/dockerfile, dockerfile)	3m 41s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	3m 38s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	3m 29s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 14s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	3m 42s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 51s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 47s
test / run (oci, ./frontend/dockerfile, integration)	3m 34s
test / run (oci, ./frontend/dockerfile, dockerfile)	3m 35s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 43s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 37s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 31s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	3m 40s
image	0s
release	0s
	59m 29s

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