You cannot - perhaps more correct, should not - do that with an encrypted root.
What you want to achieve can only be done using un-encrypted boot with extra space for the iso.
This topic explains how to achieve this [root tip] [How To] Use Calamares to install encrypted root using unencrypted boot.
Please be aware that you need to increase the size of the boot partition to be big enough or use an extra partition to store the ISO file.
If you decide to use /boot - depending on the size of the ISO - must be 4GiB as minimum.
If the inside of your luks container is tomb-stoned for any reason you have a completely unusable system - if you have the ISO outside on a separate partition you have more flexibility.