frontend/dockerfile: BFlags.Parse: include flag with "--" prefix in errors #4954

Job	Run time
build (windows/amd64)	1m 42s
build (freebsd/amd64)	1m 46s
test-freebsd-amd64	2m 42s
test-windows-amd64 (containerd, ./client)	3m 44s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 15s
test-windows-amd64 (containerd, ./worker/containerd)	3m 19s
test-windows-amd64 (containerd, ./solver)	2m 43s
test-windows-amd64 (containerd, ./frontend)	2m 52s
test-windows-amd64 (containerd, ./frontend/dockerfile)	12m 59s
test-windows-amd64 (./..., 1)	12m 29s
	46m 31s

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