Access other computer network only on vagrant/virtual box host from within docker container

I have a lab environment that is setup using vagrant. It consists of 3 computers, two application servers with docker 1.4.1 installed and one database server with postgres 9.3 installed. All machines are running Centos 6.6.

Set up your environment with a host-only private network. This is the vagrant file

Vagrant.configure('2') do |config|
  config.vm.box = "centos-6.0-updated"

  {
    'db'    => '10.17.33.10',
    'app1'   => '10.17.33.11',
    'app2' => '10.17.33.12',
  }.each do |short_name, ip|
    config.vm.define short_name do |host|
      host.vm.network 'private_network', ip: ip
      host.vm.hostname = "#{short_name}.my.dev"
    end
  end
end

I found that when I go inside the container on app1 or app2, I can't access the database server. I think the problem is that vagrant/virtualbox's host-only private network uses addresses in the 127.0.0.x range. On the host, vagrant configures the loopback interface to handle sending requests to every computer on the network. However, in the container, because this interface is not configured, the container treats all 127.0.0.x requests as requests to localhost and sends them back to itself.

Is there any alternative configuration I can set on vagrant side or docker side to mitigate this issue? In short, I want a container on my app server to communicate with the db server in a do nothing environment. Note that the database is installed directly on the db-host, not running inside a docker container. Also, this is to mimic a production environment where vagrant won't be used, so I would expect all docker changes to work in a more normal network situation as well.