Hi,
What you’re trying to do is reasonable, yes. Express Plus is reasonably agnostic about where it runs past the initial boot, and I’m always delighted to have people ask questions about restoring backups before they’re needed.
And you were correct to suspect EBS as the reason for the shenanigans; I set it up that way to take advantage of encrypted volumes with managed keys. It’s not special and if you establish the /mnt/docker symlink it should work just fine; alternately, you can tell Duplicity to use a different path to restore. https://askubuntu.com/questions/520152/extract-a-folder-from-duplicity-archive looks like it discusses this.