[...]
 ROOT_RO=/mnt/root-ro
 ROOT_RW=/mnt/root-rw
 ROOT_RW_UPPER=${ROOT_RW}/upper
 ROOT_RW_WORK=${ROOT_RW}/work
 rootmnt=/root
 [...]
 mount -t tmpfs tmpfs-root ${ROOT_RW}
 [...]
 # root is mounted on ${rootmnt}, move it to ${ROOT_RO}.
 mount -o move ${rootmnt} ${ROOT_RO}
 [...]
 # mount virtual fs ${rootmnt} with rw-fs ${ROOT_RW} on top or ro-fs ${ROOT_RO}.
 mount -t overlay -o \
         lowerdir=${ROOT_RO},upperdir=${ROOT_RW_UPPER},workdir=${ROOT_RW_WORK} \
         overlay ${rootmnt}
 [...]
 # move mount from ${ROOT_RO} to ${rootmnt}${ROOT_RO}
 mount -o move ${ROOT_RO} ${rootmnt}${ROOT_RO}
 # move mount from ${ROOT_RW} to ${rootmnt}${ROOT_RW}
 mount -o move ${ROOT_RW} ${rootmnt}${ROOT_RW}