Script to build an iso file from the macOS installer.

CreateMacOSInstallCD

#!/bin/bash

set -e # exit on error

#Everything in put in a function so "return" will work even if this script is not sourced
function do_it()
{

	if [ $(ls -1dq "Install macOS "*.app | wc -l) -gt 1 ]
	then
		echo More than one Install macOS ...app in this folder
		return 1
	fi
	if [ $(ls -1dq "Install macOS "*.app | wc -l) -eq 0 ]
	then
		if [ $(ls -1dq "Install OS X "*.app | wc -l) -gt 1 ]
		then
			echo More than one Install OS X ...app in this folder
			return 1
		else
			if [ $(ls -1dq "Install OS X "*.app | wc -l) -eq 0 ]
			then
				echo No OS X or macOS installer found in this folder
				return 1
			else
				install_app=$(ls -1dq "Install OS X "*.app)
			fi
		fi
	else
		install_app=$(ls -1dq "Install macOS "*.app)
	fi
	dmg_name=${install_app%".app"}

	echo install_app=$install_app
	echo dmg_name="$dmg_name"

	if ! [ -f "$dmg_name".dmg ]
	then
		# Create a Disk Image
		size=$(du -gd0 "$install_app"/ | cut -f1)
		((size=size+1))
		echo '-------> Create dmg of size ' ${size}
		hdiutil create -o "$dmg_name" -size ${size}G -volname "$dmg_name" -layout SPUD -fs HFS+J
	fi

	# Mount this Image to macOS
	hdiutil attach "$dmg_name".dmg -noverify
	# Use Createinstallmedia Tool to create a Installer Image
	sudo ./"$install_app"/Contents/Resources/createinstallmedia --volume "$(sparsebundle_mountpoint "$dmg_name".dmg)" --nointeraction

	# Unmount Volumes
	if [ -f "$install_app/Contents/SharedSupport/SharedSupport.dmg" ]; then
		mountpoint="$(sparsebundle_mountpoint "$install_app/Contents/SharedSupport/SharedSupport.dmg")"
		! [ -z "$mountpoint" ] && hdiutil eject "$mountpoint"
	fi
	if [ -f "$dmg_name".dmg ]
	then
		dmg_mountpoint="$(sparsebundle_mountpoint "$dmg_name".dmg)"
		if ! [ -z "$dmg_mountpoint" ]
		then
			if [ -f "$dmg_mountpoint"/"$install_app"/Contents/SharedSupport/SharedSupport.dmg ]; then
				mountpoint="$(sparsebundle_mountpoint "$dmg_mountpoint"/"$install_app"/Contents/SharedSupport/SharedSupport.dmg)"
				! [ -z "$mountpoint" ] && hdiutil eject "$mountpoint"
			fi
		  hdiutil eject "$dmg_mountpoint"
		fi
	fi


	# Convert the bigsur.dmg to a bigsur.iso for Virtual Machine
	hdiutil convert "$dmg_name".dmg -format UDTO -o "$dmg_name".cdr
	# Move and Rename bigsur Image to Desktop
	mv "$dmg_name".cdr "$dmg_name".iso
	rm "$dmg_name".dmg

}

do_it "$@"

Works from Mavericks.
Place this script in a folder where there is a mac OS installer (Install macOS... .app or Install OS X... .app) and run. You'll find an iso with the same name as the installer app.
The iso can be used for install in virtual machines.