Prevent Ubuntu asking for the CD to install packages

Recent releases of Ubuntu have a feature where the installation CD
can be used as a repository for installing software, just like any
repository on the web.

The advantage of this, obviously, is it means that you can save
bandwidth for some packages that haven’t been updated since the
release, or even sometimes be able to install new packages without an
internet connection at all. The main disadvantage - the CD has to be in
the drive.

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