Easy and is works:
truncate --size=3000M system.img mke2fs -t ext4 -L / system_rw.img sudo mount system_rw.img sudo cp -R --preserve=all /path_to_the_ronly_system.img_mountpoint/. /system_rw_mountpoint
Easy and is works:
truncate --size=3000M system.img mke2fs -t ext4 -L / system_rw.img sudo mount system_rw.img sudo cp -R --preserve=all /path_to_the_ronly_system.img_mountpoint/. /system_rw_mountpoint