Remove apache from the image and run a unicorn instance

Hi,

I think you have done a great job with this docker image. It has been inspiring and I've learnt a lot about how to create docker images properly.

I have my current redmine installation running with unicorn, and I like it, so since I'm learning docker, I thought it could be a good idea to modify this project so that instead of running with Apache, it does with a unicorn process.

I don't know ruby, I'm a django programmer so I have little experience with RoR environments, and as I said before, I'm just learning docker.

I don't expect you to incorporate this changes, just share with you my work, and learn more about it.

My goal is to modify the project so it also accepts postgresql databases. I'll work on that next.