Lock ops based on computed key rather than LLB digest #245

Job	Run time
prepare	1s
test / prepare	5m 16s
test / run (containerd, ./frontend/dockerfile, integration)	4m 50s
test / run (containerd, ./frontend/dockerfile, dockerfile)	4m 6s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	3m 42s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	3m 45s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	3m 37s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	4m 40s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 57s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 21s
test / run (oci, ./frontend/dockerfile, integration)	2m 26s
test / run (oci, ./frontend/dockerfile, dockerfile)	4m 25s
test / run (oci-rootless, ./frontend/dockerfile, integration)	3m 18s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	3m 30s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 37s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 43s
image	0s
release	0s
	1h 3m 14s

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