feat: allow setting platform for scratch image #5321

Job	Run time
prepare	1s
test / prepare	2m 4s
test / run (containerd, ./frontend/dockerfile, integration)	5m 16s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 33s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	5m 51s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	7m 53s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 29s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	7m 31s
test / run (containerd-1.7, ./frontend/dockerfile, integration)	6m 7s
test / run (containerd-1.7, ./frontend/dockerfile, dockerfile)	7m 35s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	6m 37s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 38s
test / run (oci, ./frontend/dockerfile, integration)	5m 25s
test / run (oci, ./frontend/dockerfile, dockerfile)	6m 53s
test / run (oci-rootless, ./frontend/dockerfile, integration)	5m 24s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	12m 55s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	5m 0s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	12m 51s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 59s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	11m 47s
image (mainline)	5m 30s
image (labs)	5m 51s
scout	0s
release	0s
	2h 26m 10s

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