New provider: Docker

This is the code from the [docker-provider](https://github.com/fgrehm/docker-provider) project, updated to work with Vagrant 1.5 new features and with support for Docker 0.9+ only.

Here's what I installed on my Ubuntu 13.10 machine:

```
$ docker info
Containers: 2
Images: 131
Storage Driver: aufs
 Root Dir: /home/fabio/docker/aufs
 Dirs: 135
Execution Driver: native-0.1
Kernel Version: 3.11.0-18-generic
WARNING: No swap limit support

$ docker version
Client version: 0.9.1
Go version (client): go1.2.1
Git commit (client): 3600720
Server version: 0.9.1
Git commit (server): 3600720
Go version (server): go1.2.1
Last stable version: 0.9.1
```

Here are the relevant sections from `docker-provider` README for future documentation and ideas for what do to next.

---------------------

## Features

* Support for Vagrant's `up`, `destroy`, `halt`, `reload` and `ssh` commands
* Port forwarding
* Synced / shared folders support
* Set container hostnames from Vagrantfiles
* Provision Docker containers with any built-in Vagrant provisioner (as long as the container has a SSH server running)

### Initial setup

The plugin requires Docker's executable to be available on current user's `PATH` and that the current user has been added to the `docker` group since we are not using `sudo` when interacting with Docker's CLI. For more information on setting this up please check [this page](http://docs.docker.io/en/latest/installation/ubuntulinux/#giving-non-root-access).

### `vagrant up`

On its current state, the plugin is not "user friendly" and won't provide any kind of feedback about the process of downloading Docker images, so before you add a `docker-provider` [base box](http://docs.vagrantup.com/v2/boxes.html) it is recommended that you `docker pull` the associated base box images prior to spinning up `docker-provider` containers (otherwise you'll be staring at a blinking cursor without any progress information for a while).

```sh
docker pull fgrehm/vagrant-ubuntu:precise
bundle exec vagrant box add precise64 http://bit.ly/vagrant-docker-precise
bundle exec vagrant init precise64
bundle exec vagrant up --provider=docker
```

Under the hood, that base box will [configure](#configuration) `docker-provider` to use the [`fgrehm/vagrant-ubuntu:precise`](https://index.docker.io/u/fgrehm/vagrant-ubuntu/) image that approximates a standard Vagrant box (`vagrant` user, default SSH key, etc.) and you should be good to go.


## Using custom images

If you want to use a custom Docker image without creating a Vagrant base box, you can use a "dummy" box and configure things from your `Vagrantfile` like in [vagrant-digitalocean](https://github.com/smdahlen/vagrant-digitalocean#configure) or [vagrant-aws](https://github.com/mitchellh/vagrant-aws#quick-start):

```ruby
Vagrant.configure("2") do |config|
  config.vm.box = "dummy"
  config.vm.box_url = "http://bit.ly/vagrant-docker-dummy"
  config.vm.provider :docker do |docker|
    docker.image = "your/image:tag"
  end
end
```


## Configuration

This provider exposes a few provider-specific configuration options that are passed on to `docker run` under the hood when the container is being created:

* `image` - Docker image to run (required)
* `privileged` - Give extended privileges to the container (defaults to false)
* `cmd` - An array of strings that makes up for the command to run the container (defaults to what has been set on your `Dockerfile` as `CMD` or `ENTRYPOINT`)
* `ports` - An array of strings that makes up for the mapped network ports
* `volumes` - An array of strings that makes up for the data volumes used by the container

These can be set like typical provider-specific configuration:

```ruby
Vagrant.configure("2") do |config|
  # ... other stuff

  config.vm.provider :docker do |docker|
    docker.image      = 'fgrehm/vagrant-ubuntu-dind:precise'
    docker.privileged = true
    docker.cmd        = ['/dind', '/sbin/init']

    docker.ports   << '1234:22'
    docker.volumes << '/var/lib/docker'
  end
end
```


## Networks

Networking features in the form of `config.vm.network` are not supported with `docker-provider` apart from [forwarded ports](). If any of [`:private_network`](http://docs.vagrantup.com/v2/networking/private_network.html) or [`:public_network`](http://docs.vagrantup.com/v2/networking/public_network.html) are specified, Vagrant **won't** emit a warning.

The same applies to changes on forwarded ports after the container has been created, Vagrant **won't** emit a warning to let you know that the ports specified on your `Vagrantfile` differs from what has been passed on to `docker run` when creating the container.

_At some point the plugin will emit warnings on the scenarios described above, but not on its current state. Pull Requests are encouraged ;)_


## Synced Folders

There is support for synced folders on the form of [Docker volumes](http://docs.docker.io/en/latest/use/working_with_volumes/#mount-a-host-directory-as-a-container-volume) but as with forwarded ports, you won't be able to change them after the container has been created. [NFS](http://docs.vagrantup.com/v2/synced-folders/nfs.html) synced folders are also supported (as long as you set the `privileged` [config](#configuration) to true so that `docker-provider` can mount it on the guest container) and are capable of being reconfigured between `vagrant reload`s (different from Docker volumes).

This is good enough for all built-in Vagrant provisioners (shell, chef, and puppet) to work!

_At some point the plugin will emit warnings when the configured `Vagrantfile` synced folders / volumes differs from the ones used upon the container creation, but not on its current state. Pull Requests are encouraged ;)_


## Box format

The box format is basically just the required `metadata.json` file along with a `Vagrantfile` that does default settings for the provider-specific configuration for this provider.


## Available base boxes

| LINK | DESCRIPTION |
| ---  | ---         |
| http://bit.ly/vagrant-docker-precise | Ubuntu 12.04 Precise x86_64 with Puppet and Chef preinstalled and configured to run `/sbin/init` |
| http://bit.ly/vagrant-docker-precise-dind | Ubuntu 12.04 Precise x86_64 based on the box above and ready to run [DinD](https://github.com/jpetazzo/dind) |


## Limitations

As explained on the [networks](#networks) and [synced folder](#synced-folders) sections above, there are some "gotchas" when using the plugin that you need to have in mind before you start to pull your hair out.

For instance, forwarded ports, synced folders and containers' hostnames will not be reconfigured on `vagrant reload`s if they have changed and the plugin **_will not give you any kind of warning or message_**. As an example, if you change your Puppet manifests / Chef cookbooks paths (which are shared / synced folders under the hood), **_you'll need to start from scratch_** (unless you make them NFS shared folders). This is due to a limitation in Docker itself as we can't change those parameters
after the container has been created.
If we want to create a docker machine with a static IP address --- would it require patching this create method?