gateway: ensure llb digests are deterministic when sent by frontends #4849

Job	Run time
prepare	1s
test / prepare	2m 4s
test / run (containerd, ./frontend/dockerfile, integration)	4m 51s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 5s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 5s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 53s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 8s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 46s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 10s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 24s
test / run (oci, ./frontend/dockerfile, integration)	4m 30s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 3s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 52s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	6m 29s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 52s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 49s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 6s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 36s
image (mainline)	5m 7s
image (labs)	5m 29s
scout	0s
release	0s
	1h 46m 20s

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