Cache apt packages used on acceptance specs to make them run faster

spec/Vagrantfile

@@ -1,6 +1,16 @@
 # -*- mode: ruby -*-
 # vi: set ft=ruby :
 
+def local_apt_cache(box_name)
+  cache_dir = File.join(File.expand_path(Vagrant::Environment::DEFAULT_HOME),
+                        'cache',
+                        'apt',
+                        box_name)
+  partial_dir = File.join(cache_dir, 'partial')
+  FileUtils.mkdir_p(partial_dir) unless File.exists? partial_dir
+  cache_dir
+end
+
 Vagrant.require_plugin 'vagrant-lxc'
 
 Vagrant.configure("2") do |config|
@@ -10,4 +20,7 @@ Vagrant.configure("2") do |config|
   config.vm.box_url = 'http://dl.dropbox.com/u/13510779/lxc-quantal64-2013-04-10.box'
   # Uncomment to test boxes built locally:
   # config.vm.box_url = '../boxes/output/lxc-quantal64.box'
+
+  cache_dir = local_apt_cache(config.vm.box)
+  config.vm.synced_folder cache_dir, "/var/cache/apt/archives", id: "vagrant-apt-cache"
 end