Thanks for your report, I have already found the cause of the issue and I'm going to add a fix in the next release.
As a workaround, you can manually add the missing root account with :
# Replace YOUR-PASSWORD with the password set in /etc/mysql/conf.d/my.cnf
mysql -e "use mysql; GRANT ALL PRIVILEGES on *.* TO 'root'@'127.0.0.1' IDENTIFIED by 'YOUR-PASSWORD' WITH GRANT OPTION"
mysql -e "flush privileges;"