Multi-arch images are not supported

[anguslees]

Known issue, but I didn't see an open bug tracking it.

ie:

container-structure-test/cmd/container-structure-test/app/cmd/test.go

Lines 127 to 129 in ec9020e

	if desc.MediaType.IsIndex() {
	logrus.Fatal("multi-arch images are not supported yet.")
	}