Add file for non-Linux placeholder.

internal/app/wwctl/container/exec/child/main.go

@@ -1,4 +1,4 @@
-// + build linux
+// +build linux
 
 package child
 

internal/app/wwctl/container/exec/child/non-Linux.go

@@ -0,0 +1,14 @@
+// +build !linux
+
+package child
+
+import (
+	"github.com/hpcng/warewulf/internal/pkg/wwlog"
+	"github.com/spf13/cobra"
+)
+
+func CobraRunE(cmd *cobra.Command, args []string) error {
+	wwlog.Printf(wwlog.ERROR, "This command does not work on non-Linux hosts\n")
+
+	return nil
+}