dockerfile: update args definitions to llb.EnvList #4330

Job	Run time
prepare	1s
test / prepare	2m 0s
test / run (containerd, ./frontend/dockerfile, integration)	5m 31s
test / run (containerd, ./frontend/dockerfile, dockerfile)	5m 6s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 49s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	5m 35s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 17s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	5m 5s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 59s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 16s
test / run (oci, ./frontend/dockerfile, integration)	4m 54s
test / run (oci, ./frontend/dockerfile, dockerfile)	4m 33s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 56s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	4m 42s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 24s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 3s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 26s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	4m 32s
image	0s
scout	0s
release	0s
	1h 22m 9s

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