I have never done this before, but a look at the man page tells the --root=/foo command makes /foo the root directory for the entire installation process.
So 2 things could be happening: If the install package is located in /foo/blah.deb, you should install blah.deb, not /foo/blah.deb.
Secondly, dpkg expects a complete directory tree with directories like /usr /bin /lib /etc and so on. Once you set root to /foo, there must be a directory structure below /foo, thus /foo/usr, /foo/var. And that might be the hardest, I don't know if dpkg creates them.
Besides, any package running expects a correctly set up /tmp, /var, /bin, /etc and whatnot. If you don't have them, the program doesn't run.
jlinkels
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