...attach the read-only image as with the shadow option:

From https://wiki.afp548.com/index.php/Mount_DMG_Read_Write:

...attach the read-only image as with the shadow option:

    hdiutil attach -owners on your.dmg -shadow

After typing this command, the image will be attached as a new device to your operating system. Usually this is shown from the command line.

It's the -shadow option that enables this.

attach the read-only image as with the shadow option:

hdiutil attach -owners on your.dmg -shadow