frontend/dockerfile: BFlags.Parse: include flag with "--" prefix in errors #4448

Job	Run time
prepare	1s
test / prepare	1m 56s
test / run (containerd, ./frontend/dockerfile, integration)	4m 41s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 19s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 58s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 40s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 38s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 0s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 20s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 46s
test / run (oci, ./frontend/dockerfile, integration)	4m 20s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 30s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 10s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 12s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 22s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 44s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 42s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 25s
image (mainline)	4m 49s
image (labs)	4m 37s
release	0s
scout	0s
	1h 37m 10s

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