Update Dockerfile

Details of the error:

```
$ sudo docker build -t ngx .
Uploading context 112.6 kB
Uploading context
Step 1 : FROM ubuntu:12.04
 ---> 9cd978db300e
# Skipping unknown instruction MAINTENER
Step 2 : RUN dpkg-divert --local --rename --add /sbin/initctl
 ---> Running in 828e0fee043d
Leaving 'local diversion of /sbin/initctl to /sbin/initctl.distrib'
 ---> 32e919e338c3
Step 3 : RUN ln -s /bin/true /sbin/initctl
 ---> Running in f2409865ebe4
ln: failed to create symbolic link `/sbin/initctl': File exists
2014/02/19 10:18:16 The command [/bin/sh -c ln -s /bin/true /sbin/initctl] returned a non-zero code: 1
```