Several mentions here of using overlay file-system via the RaspberryPi specific raspi-config options. However, where-ever systemd is the init one can do it via

Several mentions here of using overlay file-system via the RaspberryPi specific raspi-config options. However, where-ever systemd is the init one can do it via adding to the kernel command line systemd.volatile=overlay "If this setting is set to "overlay" the root file system is set up as "overlayfs" mount combining the read-only root directory with a writable "tmpfs", so that no modifications are made to disk, but the file system may be modified nonetheless with all changes being lost at reboot." See man 7 kernel-command-line for the other volatile mode options.