Unlike the other operating systems which have a bash script that runs
on Linux and scripts qemu to prepare a GCE image, the Windows image
builder image is prepared by hand, following this list of
instructions:

-- In the GCE project's Networks > Default network, edit the "RDP" rule and
   set its "Target tag" to "allow-rdp".  (VMs will require the "allow-rdp" tag
   to have any RDP access from the outside)

-- in GCE, create a new Windows 2008 server instance. Set its initial
   username to "wingopher" and the password to something. Be sure to
   uncheck "Delete the boot disk when instance terminates".

-- boot it. first boot is slow.

-- Connect with RDP to it. (I used the OS X Microsoft Remote Desktop
   in the Mac App Store)

-- In initial “Initial Configuration Tasks” window:
   * disable automatic installation of updates. They’ll just interrupt
     the builds later. And we don't care about security since this isn't
     going to be Internet-facing. No ports will be accessible
   * disable Windows Error Report (“Not participating”)
   * disable the Windows firewall (GCE provides its own)

-- Enable auto-login:
   * Start > Run > Open: "control userpasswords2" [OK]
   * Uncheck the "Users must enter a user name..." box (it already was
     unchecked for me)
   * Click to highlight the sole user (this makes it be the auto-login user)
   * Press "OK"
   * Enter password twice.

-- Prevent password expiration. This bit us once, and I'm not sure this is yet
   sufficient, but:
      User Accounts (from above) > Advanced (tab) > Advanced (button) >
      [lusrmgr]: for all three users, disable password expiration (right click
      and then Properties and it's a checkbox)

-- Download winstrap.
   * Bring up cmd.exe (Start > Run > "cmd" [OK])
   * Copy/paste (Right click: paste) this into cmd.exe:

     bitsadmin /transfer mydownloadjob  /download /priority normal https://storage.googleapis.com/winstrap/winstrap-2015-02-11-046f2a8.exe c:\users\wingopher\Desktop\winstrap.exe

   * It should appear on the desktop.

-- double click winstrap. type "go" and press <enter>. abort it once
   it's downloaded everything. (close the window)

-- make the Buildlet start on boot:
   * Start > All Programs > right click "Startup"
   * Right-mouse-drag "Start Buildlet" from the desktop (from winstrap) to the
     Startup folder and "Create shortcuts here"

-- Install wix35 (ignore the warning about notinstalling "Votive" something)

-- Install git with middle option: "Use Git from Windows Command Prompt",
   otherwise all the defaults.

-- click yes yes yes on the tdm64-gcc installer question defaults.
   Then do the tdm64-gcc installer again but do 32-bit this time. You
   should then have two TDM installs at c:\TDM-GCC-64 and
   c:\TDM-GCC-32.  Don’t pick other locations. The gobuilder adds
   those to your path.

-- disable GCE service so things boot quickly and don't kill the user
   we've configured on boot.
   - Run Regedit,
   - Go to the registry entry "HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services"
   - delete the Google Compute Engine folder.

-- disable GCEStartupTask in the "Task Scheduler", for the same reason.
   Maybe only one of these two is required. Might as well do both.

-- shutdown the machine

-- delete the GCE image (but not its boot disk)

-- Go to GCE Compute > Images > New Image and create an image named "windows-buildlet" with
   "Source Type" of "Disk" and pick the Source Disk from the previously-deleted image.
   You can now delete the old disk if you want.


** Misc docs:

http://www.win2008workstation.com/configure-auto-logon/
https://cloud.google.com/compute/docs/operating-systems/windows
  -- notably "windows-startup-script-cmd" ala:
     gcloud compute instances add-metadata <INSTANCE> --zone <ZONE> --metadata windows-startup-script-cmd="net user <USERNAME> <PASSWORD>"
https://github.com/golang/winstrap
http://golang.org/s/windowsbuilderhowto