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Using ISO image of installer DVD as repository - How?

With the first DVD of Debian 9.8.0 I did:
   dd if=/dev/cdrom of=dvd.iso bs=4M
I edited sources.list to read
   deb file:///home/richard/dvd.iso stretch main contrib trusted=yes

When running Synaptic's
    Edit->Reload Package Information
I receive an error message:
The repository 'file:/home/richard/dvd.iso stretch Release'
does not have a Release file.

What is the problem?