dockerfile: fix syntax forwarding for files with BOM #5800

Job	Run time
sandbox-build (linux/amd64)	6m 46s
build (windows/amd64)	1m 50s
sandbox-build (linux/arm64)	6m 39s
build (freebsd/amd64)	1m 47s
test-windows-amd64 (containerd, ./client)	6m 20s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 59s
test-freebsd-amd64	2m 26s
test-windows-amd64 (containerd, ./worker/containerd)	3m 19s
test-windows-amd64 (containerd, ./solver)	2m 7s
test-windows-amd64 (containerd, ./frontend)	2m 58s
test-windows-amd64 (containerd, ./frontend/dockerfile)	36m 53s
test-windows-amd64 (./..., 1)	3m 34s
	1h 17m 38s

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