Creating rootfs using multistrap from local repository
I have studied multistrap for sometime now but I don't see this question being asked. Maybe it is invalid but I don't see why.
I made an initial rootfs using source=http://ftp.uk.debian.org/emdebian/grip and save the source packages in a separate directory using 'retainsources' keyword. Is it possible to create the same rootfs using packages saved in the 'retainsources' directory? This would avoid downloading from the remote server again.
I see some mentions of "copy://" and "file://" but not sure if it is use for this purpose. If it is, how exactly do I used it?