Lock ops based on computed key rather than LLB digest #246

Job	Run time
prepare	1s
test / prepare	1m 45s
test / run (containerd, ./frontend/dockerfile, integration)	1m 29s
test / run (containerd, ./frontend/dockerfile, dockerfile)	1m 28s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	2m 56s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	3m 0s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	2m 59s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	2m 59s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	2m 57s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	2m 58s
test / run (oci, ./frontend/dockerfile, integration)	2m 58s
test / run (oci, ./frontend/dockerfile, dockerfile)	2m 59s
test / run (oci-rootless, ./frontend/dockerfile, integration)	2m 55s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	2m 37s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	2m 57s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	2m 58s
image	1s
release	1s
	39m 58s

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