Installing Docker on Ubuntu provided by Rackspace is pretty straightforward, and you should mostly be able to follow the Ubuntu installation guide.
However, there is one caveat:
If you are using any Linux not already shipping with the 3.8 kernel you will need to install it. And this is a little more difficult on Rackspace.
Rackspace boots their servers using grub's menu.lst and does not like non virtual packages (e.g., Xen compatible) kernels there, although they do work. This results in update-grub not having the expected result, and you will need to set the kernel manually.
Do not attempt this on a production machine!
# update apt $ apt-get update # install the new kernel $ apt-get install linux-generic-lts-raring
Great, now you have the kernel installed in /boot/, next you need to make it boot next time.
# find the exact names $ find /boot/ -name '*3.8*' # this should return some results
Now you need to manually edit /boot/grub/menu.lst, you will find a section at the bottom with the existing options. Copy the top one and substitute the new kernel into that. Make sure the new kernel is on top, and double check the kernel and initrd lines point to the right files.
Take special care to double check the kernel and initrd entries.
# now edit /boot/grub/menu.lst $ vi /boot/grub/menu.lst
It will probably look something like this:
## ## End Default Options ## title Ubuntu 12.04.2 LTS, kernel 3.8.x generic root (hd0) kernel /boot/vmlinuz-3.8.0-19-generic root=/dev/xvda1 ro quiet splash console=hvc0 initrd /boot/initrd.img-3.8.0-19-generic title Ubuntu 12.04.2 LTS, kernel 3.2.0-38-virtual root (hd0) kernel /boot/vmlinuz-3.2.0-38-virtual root=/dev/xvda1 ro quiet splash console=hvc0 initrd /boot/initrd.img-3.2.0-38-virtual title Ubuntu 12.04.2 LTS, kernel 3.2.0-38-virtual (recovery mode) root (hd0) kernel /boot/vmlinuz-3.2.0-38-virtual root=/dev/xvda1 ro quiet splash single initrd /boot/initrd.img-3.2.0-38-virtual
Reboot the server (either via command line or console)
# reboot
Verify the kernel was updated
$ uname -a # Linux docker-12-04 3.8.0-19-generic #30~precise1-Ubuntu SMP Wed May 1 22:26:36 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux # nice! 3.8.
Now you can finish with the Ubuntu instructions.