Unable to run multiple docker containers based off the same image

I'm not sure I'll be able to dig into this tonight but looks like we might need to change the docker provisioner to allow running multiple containers based off the same image.

Given the following provisioner block:

```ruby
  config.vm.provision :docker do |docker|
    docker.run "busybox", cmd: "/bin/sh -c 'while true; do date; sleep 1; done'"
    docker.run "busybox", cmd: "/bin/sh -c 'while true; do echo \"hello world\"; sleep 1; done'"
  end
```

The result of a `vagrant provision` gives us a single container on the output:

```
[default] Running provisioner: docker...
[default] Starting Docker containers...
[default] -- Container: busybox
```

And just to confirm:

```
$ docker ps -a
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS               NAMES
a52f6e95a1cb        busybox:latest      /bin/sh -c while tru   3 seconds ago       Up 2 seconds                            determined_bardeen
```

I'm not sure what would be the best approach to handle that but I think this is useful as one might want to have multiple instances of the same container around.