Lock ops based on computed key rather than LLB digest #248

Job	Run time
prepare	1s
test / prepare	3m 20s
test / run (containerd, ./frontend/dockerfile, integration)	9m 11s
test / run (containerd, ./frontend/dockerfile, dockerfile)	13m 41s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	8m 54s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	13m 32s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	8m 26s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	13m 5s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	3m 20s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	9m 33s
test / run (oci, ./frontend/dockerfile, integration)	8m 46s
test / run (oci, ./frontend/dockerfile, dockerfile)	13m 18s
test / run (oci-rootless, ./frontend/dockerfile, integration)	8m 49s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	12m 12s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	9m 9s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	54s
image	0s
release	0s
	2h 16m 11s

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