only need to support linux for running containers

MacOS cannot run containers natively. M1/M2 defaults to linux/arm64 so
the "os" field as part of --platform only needs to use linux/ as a
default with GOARCH read in for the particular CPU architecture. Same
deal with Windows, Docker on windows is virtualized Linux (AFAIU)

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

@@ -227,7 +227,7 @@ func AddTestFlags(cmd *cobra.Command) {
 	cmd.Flags().StringVarP(&opts.Driver, "driver", "d", "docker", "driver to use when running tests")
 	cmd.Flags().StringVar(&opts.Metadata, "metadata", "", "path to image metadata file")
 	cmd.Flags().StringVar(&opts.Runtime, "runtime", "", "runtime to use with docker driver")
-	cmd.Flags().StringVar(&opts.Platform, "platform", fmt.Sprintf("%s/%s", runtime.GOOS, runtime.GOARCH), "Set platform if host is multi-platform capable")
+	cmd.Flags().StringVar(&opts.Platform, "platform", fmt.Sprintf("linux/%s", runtime.GOARCH), "Set platform if host is multi-platform capable")
 	cmd.Flags().BoolVar(&opts.Pull, "pull", false, "force a pull of the image before running tests")
 	cmd.MarkFlagsMutuallyExclusive("image-from-oci-layout", "pull")
 	cmd.Flags().BoolVar(&opts.Save, "save", false, "preserve created containers after test run")