… just “open” instead of “luksOpen” will also do the job
… you do know that the word you use as the last in this command (“root” here) is totally arbitrary - you can choose whatever word you want?
If you didn’t - now you do 
It’s what the /dev/mapper/... will be named.
It’s just a name tag.
This command won’t work from within chroot.
But you didn’t say it would - you just meant to say:
… when done, reboot …
It’s probably playing it a bit safer when you first exit the chroot environment before you reboot.