dockerfile: don't allow non-Dockerfile syntax for directives #5191

Job	Run time
prepare	1s
test / prepare	18m 58s
test / run (containerd, ./frontend/dockerfile, integration)	24m 24s
test / run (containerd, ./frontend/dockerfile, dockerfile)	23m 1s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	24m 23s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	36m 25s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	24m 22s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	36m 39s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	24m 22s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	23m 2s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	20m 0s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	23m 36s
test / run (oci, ./frontend/dockerfile, integration)	23m 49s
test / run (oci, ./frontend/dockerfile, dockerfile)	41m 18s
test / run (oci-rootless, ./frontend/dockerfile, integration)	23m 42s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	45m 4s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	32m 24s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	35m 53s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	24m 2s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	36m 6s
image (mainline)	5m 33s
image (labs)	5m 51s
scout	0s
release	0s
	9h 12m 55s

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