After hours of trying and failing net-install Ubuntu 6.06 (Dapper Drake) from my local repository, I finally found out what's missing from it.
I did not include main/debian-installer in my debmirror script!
Thus, the repository is only good for installing packages into up & running Ubuntu system.
With less than 3 GB disk space remaining, I don't think I can afford to add main/debian-installer into my local repo.
Thus, having CD-installer ready is a must.