gateway: ensure llb digests are deterministic when sent by frontends #4857

Job	Run time
prepare	1s
test / prepare	1m 54s
test / run (containerd, ./frontend/dockerfile, integration)	4m 52s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 0s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 4s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 57s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 22s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	6m 55s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 10s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 22s
test / run (oci, ./frontend/dockerfile, integration)	4m 16s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 1s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 34s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	6m 3s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 56s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 42s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 6s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 14s
image (mainline)	5m 21s
image (labs)	5m 32s
release	0s
scout	0s
	1h 45m 22s

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