dockerfile: fix copy non-octal build tag #5122

Job	Run time
prepare	1s
test / prepare	2m 19s
test / run (containerd, ./frontend/dockerfile, integration)	4m 55s
test / run (containerd, ./frontend/dockerfile, dockerfile)	6m 54s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 55s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 51s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	4m 57s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 5s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	4m 57s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 26s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 11s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 30s
test / run (oci, ./frontend/dockerfile, integration)	5m 0s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 3s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 29s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	6m 0s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 48s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 48s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 29s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	5m 59s
image (mainline)	5m 15s
image (labs)	5m 14s
release	0s
scout	0s
	1h 57m 6s

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