FROM must be the first non-comment instruction in the Dockerfile

I encountered an error bellow:

```bash
$ vagrant up
...
[default] Running provisioner: docker...
[default] Installing Docker (latest) onto machine...
[default] Starting Docker containers...
[default] -- Container: elasticsearch
[default] -- Container: kibana
[default] Running provisioner: shell...
[default] Running: inline script
stdin: is not a tty
Uploading context 81.92 kB
Step 1 : MAINTAINER riywo "https://github.com/riywo"
2013/12/15 06:57:31 Please provide a source image with `from` prior to commit
[default] Running provisioner: docker...
[default] Starting Docker containers...
[default] -- Container: td-agent
The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!


          rm -f /var/lib/vagrant/cids/b197f5c90634a16396fb8eeeb8432e3e5f15d07e
          docker run -cidfile=/var/lib/vagrant/cids/b197f5c90634a16396fb8eeeb8432e3e5f15d07e -d -name td-agent -p 24224:24224 -link elasticsearch:es td-agent 
        

Stdout from the command:



Stderr from the command:

stdin: is not a tty
Unable to find image 'td-agent' (tag: latest) locally
Pulling repository td-agent
2013/12/15 06:57:34 HTTP code: 404

```

Since I found the following description, I have fixed this problem.

http://docs.docker.io/en/latest/use/builder/

> FROM must be the first non-comment instruction in the Dockerfile.