setup-sshd and root login
By default setup-alpine does not create additional users apart from the root user. It does, however, invoke setup-sshd to configure an SSH daemon. When dropbear is selected as an SSH daemon it seems that root logins are allowed by default. However, if OpenSSH is selected root logins are deactivated by default (PermitRootLogin), i.e. the user would not be able to login after reboot.
I would suggest asking the user—independent of the selected SSH daemon—whether or not root logins should be allowed and adjusting the default openssh/dropear configuration accordingly.