Nginx Dockerfile

## WHAT
Image Server using Nginx with ngx_small_light module in Docker Container.

### Features

* resize images
* crop images
* simple proxy (not resizing, cropping)
* change image format (Example: `png` to `webp`)

### How to build 
This compile step is heavily inspired from [miketheman/nginx](https://github.com/miketheman/nginx) (Chef cookbook).

## TODO

* [x] log to stdout
* [x] compile with http_stub_status_module
* [x] compile with ngx_small_light module
* [x] set s3 host via env
* [x] refactor Dockerfile
* [x] add favicon
* [x] build ImageMagick with WebP support
* [x] add README
* ~~add serverspec?~~
* [x] add infrataster(behavior test)
* [x] CI on wercker 
* [ ] Vagrantfile + user-data

## REF

* [[WIP] Install nginx with ngx_small_light #1](https://github.com/wantedly/image-server/pull/1)

### env in nginx.conf

```
Syntax:	env variable[=value];
Default:	
env TZ;
Context:	main
```

> By default, nginx removes all environment variables inherited from its parent process except the TZ variable. This directive allows preserving some of the inherited variables, changing their values, or creating new environment variables.
http://nginx.org/en/docs/ngx_core_module.html

With Perl

>Using Lua was our preferred approach, since we have OpenResty. If you can't use Lua, a second solution involves using Perl. The first part is similar; you must declare the variables he uses using env:
>
```
env API_KEY;
```
After that, you can combine perl_set and some Perl to do the same thing as before:
>
```
http {
  ...
  server {
    location / {
      perl_set $api_key 'sub { return $ENV{"API_KEY"}; }';
      ...
    }
  }
}
```
You will need to have the ngx_http_perl_module module enabled in order to be able to use this technique.
https://docs.apitools.com/blog/2014/07/02/using-environment-variables-in-nginx-conf.html

### log to stdout
http://stackoverflow.com/questions/22541333/have-nginx-access-log-and-error-log-log-to-stdout-and-stderr-of-master-process

### ImageMagick with WebP

* http://d.hatena.ne.jp/yoya/20130424/webp
* http://askubuntu.com/questions/251950/imagemagick-convert-cant-convert-to-webp
file下にこれを置かずに、別のディレクトリに置いて、そこをディレクトリごとignoreした方がいいような気がした。
こっちとしてはfile下の他の2つは必要でこれは必要ないんだなって理解するのに時間がかかった気がした。