Physical-to-Docker conversion script ... as in, these are the steps I went through, but this script is not something you want to run on any machine you care about. I've removed employer-specific stuff from this copy.

p2d.sh

#!/bin/bash
#
# This is really just notes, but I put them in a shell script so I get
# free highlighting and easy cut/paste (in and out).
#

echo "Don't run this script! It's meant to be read. It will destroy your system!"
exit 1

RAW="lucid-amd64-cdh3u4-raw"
NAME="lucid-amd64-cdh3u4"
SOURCE="hdp-c1mr9.foobar.com"
KEEPUSERS="mapred"

# I use btrfs so I don't have to re-clone if I screw up
btrfs subvolume create $RAW
rsync -avxe ssh root@$SOURCE:/ $RAW

# make a snapshot to do the actual setup
btrfs subvolume snapshot $RAW $NAME

chroot $NAME /bin/bash -l

# clean up user dirs
rm -rf /root/*
cd /home
for user in $KEEPUSERS; do mv $user .$user; done
for user in *; do userdel $user 2>/dev/null; rm -rf $user; done
for user in $KEEPUSERS; do mv .$user $user; chown -R $user $user; done

# clean up one user's RVM
rm -rf /home/mapred/.rvm/src /home/mapred/.rvm/archives

# install stuff that's already there so they don't get marked for autoremove
apt-get install -y busybox-static \
  xz-utils bsd-mailx \
  libxslt1.1 libyaml-0-2 libopenssl-ruby libruby \
  mysql-common

# remove system services that aren't required
apt-get remove \
  grub2 grub-pc grub-common linux-image linux-firmware \
  $(dpkg --list |awk '/-dev /{print $2}') \
  dnscache-run djbdns daemontools \
  smartmontools mdadm lm-sensors \
  apport at avahi-daemon cron logrotate rsyslog dbus consolekit \
  irqbalance console-setup kbd dhcpcd dmsetup \
  xinetd update-inetd \
  gsfonts-x11 libice6 libsm6 libxt6 libxtst6 x11-common xfonts-encodings xfonts-utils \
  ntp ntpdate powermgmt-base \
  dstat sysstat tcpdump \
  apport-symptoms \
  gcc gdb autoconf gcc-4.4 \
  gdisk hdparm laptop-detect \
  cpu-checker dmidecode dosfstools eject fuse-utils

# remove extra stuff that was pulled in as dependencies
apt-get autoremove -y

# clean up leftover garbage from removed packages
for pkg in $(dpkg --list |awk '/^rc/{print $2}')
do
  dpkg --purge $pkg
done

# remove old logs
logrotate -f /etc/logrotate.conf # force rotation
rm -f /var/log/*.gz /var/log/rsyslog/* /var/log/dmesg*

# delete cruft
rm -rf /etc/mdadm /etc/sv
rm -rf /lost+found
rm -rf /mnt /media
rm -rf /tmp/* /var/tmp/* /run/* /initrd
rm -rf /boot /lib/modules /lib/firmware

# remove old deploys
for app in /opt/*/*
do
  current=$(readlink $app/current)
  if [ -n "$current" ] ; then
    for reldir in $app/releases/*
    do
      if [ "$reldir" != "$current" ] ; then
        rm -rf $reldir
      fi
    done
  fi
done

# make sure basic paths are there
mkdir /run /data

# make sure some permissions are sane
chown -R root:root /lib /root

# now add some packages specific to how we use Docker
# ignore upstart since it's not going to get used
apt-get install -y runit
apt-get clean

# make sure /etc/hosts is sane
cat > /etc/hosts <<EOF
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
EOF

# unconfigure networking
echo "unconfigured" > /etc/hostname
cat > /etc/network/interfaces <<EOF
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
EOF

# reconfigure rsyslog to forward
cat > /etc/rsyslog.conf <<EOF
\$ModLoad imuxsock
\$ModLoad imklog
\$KLogPath /var/run/rsyslog/kmsg
\$FileOwner syslog
\$FileGroup adm
\$FileCreateMode 0644
\$DirCreateMode 0755
\$Umask 0022
\$PrivDropToUser syslog
\$PrivDropToGroup syslog
\$PreserveFQDN on
*.* @10.0.0.1
EOF

# then I test in LXC real quick to get my app working
cat > tester.xml <<EOF
<domain type='lxc'>
  <name>tester</name>
  <uuid>deadbeef-cafe-dead-beef-cafedeadbeef</uuid>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='static'>1</vcpu>
  <os>
    <type arch='x86_64'>exe</type>
    <init>/bin/bash</init>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/lib/libvirt/libvirt_lxc</emulator>
    <filesystem type='mount' accessmode='passthrough'>
      <source dir='$NAME'/>
      <target dir='/'/>
    </filesystem>
    <filesystem type='ram' accessmode='passthrough'>
      <source usage='0' units='KiB'/>
      <target dir='/dev/shm'/>
    </filesystem>
    <filesystem type='ram' accessmode='passthrough'>
      <source usage='0' units='KiB'/>
      <target dir='/run'/>
    </filesystem>
    <console type='pty'>
      <target type='lxc' port='0'/>
      <alias name='console0'/>
    </console>
  </devices>
</domain>
EOF

virsh -c lxc:/// define tester.xml
virsh -c lxc:/// create --console tester