ociindex: allow readonly access #5220

Job	Run time
build (windows/amd64)	1m 50s
build (freebsd/amd64)	1m 40s
test-freebsd-amd64	3m 1s
test-windows-amd64 (containerd, ./client)	4m 56s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 57s
test-windows-amd64 (containerd, ./worker/containerd)	3m 0s
test-windows-amd64 (containerd, ./solver)	2m 30s
test-windows-amd64 (containerd, ./frontend)	4m 52s
test-windows-amd64 (containerd, ./frontend/dockerfile)	40m 18s
test-windows-amd64 (./..., 1)	12m 12s
	1h 18m 16s

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