In order to do an install from completely local resources we need to mirror files that apt-mirror ignores. Specifically, we need the udebs that are used in the debian-installer. To make sure we're up-to-date I've added a little extra to the postmirror script, but I was having trouble debugging a problem with it, so I rewrote it to be a bit tidier. The following creates the administrative layer that tells the debian-installer what files to look for and where they are. It basically mirrors the packages lists in each sub-directory in the apt tree. We still need to run the udebs script to get the udebs themselves.

################################## postmirror.sh ##########################################################
#!/bin/sh -x
# the udebs script gets the actual files we need
#/mnt/repo/apt-mirror/var/udebs.sh

# A quick apt directory structure primer:
# an apt server (e.g. archive.ubuntu.com) contains repositories (e.g. trusty-backports),
# which contain archives (e.g. multiverse), which contain directories
# a complete example - http://archive.ubuntu.com/ubuntu/dists/trusty-backports/multiverse/debian-installer/

# With this in mind, we create bash 'arrays' of the structure:
# server we're syncing against
MIRROR="archive.ubuntu.com"
# repositories we're mirroring
REPOS="trusty
trusty-updates
trusty-security
trusty-proposed
trusty-backports"
# archives in repositories
ARCHIVES="main
multiverse
restricted
universe"
# installer location inside archive
DIRECTORIES="debian-installer
dist-upgrader-all
installer-amd64
installer-i386"
#where we're storing it locally
LOCALDIR="/mnt/repo/apt-mirror/mirror/archive.ubuntu.com"

for REPO in $REPOS; do
    for ARCHIVE in $ARCHIVES; do
        for DIRECTORY in $DIRECTORIES;do
            # create directory structure
            if [ ! -e "$LOCALDIR/ubuntu/dists/$REPO/$ARCHIVE/$DIRECTORY" ]; then
                mkdir -p "$LOCALDIR/ubuntu/dists/$REPO/$ARCHIVE/$DIRECTORY"
            fi
            # do the sync
            rsync --recursive --times --links --hard-links --delete --delete-after \
            rsync://$MIRROR/ubuntu/dists/$REPO/$ARCHIVE/$DIRECTORY/ $LOCALDIR/ubuntu/dists/$REPO/$ARCHIVE/$DIRECTORY
        done
    done    
done