Add nginx image

```console
root@b729e0be035c:/stackbrew/stackbrew# ./brew-cli --debug -b nginx --targets nginx git://github.com/infosiftr/stackbrew
2014-06-04 22:30:35,048 DEBUG Cloning repo_url=git://github.com/infosiftr/stackbrew, ref=nginx
2014-06-04 22:30:35,050 DEBUG folder = /tmp/tmptLaL6n
2014-06-04 22:30:35,050 DEBUG Cloning git://github.com/infosiftr/stackbrew into /tmp/tmptLaL6n
2014-06-04 22:30:35,050 DEBUG Executing "git clone git://github.com/infosiftr/stackbrew ." in /tmp/tmptLaL6n
Cloning into '.'...
remote: Counting objects: 743, done.
remote: Compressing objects: 100% (389/389), done.
remote: Total 743 (delta 407), reused 643 (delta 343)
Receiving objects: 100% (743/743), 112.31 KiB | 0 bytes/s, done.
Resolving deltas: 100% (407/407), done.
Checking connectivity... done.
2014-06-04 22:30:35,926 DEBUG Checkout ref:nginx in /tmp/tmptLaL6n
2014-06-04 22:30:35,926 DEBUG Executing "git checkout nginx" in /tmp/tmptLaL6n
Branch nginx set up to track remote branch nginx from origin.
Switched to a new branch 'nginx'
2014-06-04 22:30:35,932 DEBUG latest: git://github.com/docker-library/nginx@eca68498a6919e324e3fb728f1df014740fcc2ee

2014-06-04 22:30:35,932 DEBUG 1: git://github.com/docker-library/nginx@eca68498a6919e324e3fb728f1df014740fcc2ee

2014-06-04 22:30:35,932 DEBUG 1.7: git://github.com/docker-library/nginx@eca68498a6919e324e3fb728f1df014740fcc2ee

2014-06-04 22:30:35,932 DEBUG 1.7.1: git://github.com/docker-library/nginx@eca68498a6919e324e3fb728f1df014740fcc2ee

2014-06-04 22:30:35,933 DEBUG nginx: latest,1,1.7,1.7.1
2014-06-04 22:30:35,933 DEBUG Cloning repo_url=git://github.com/docker-library/nginx, ref=eca68498a6919e324e3fb728f1df014740fcc2ee
2014-06-04 22:30:35,933 DEBUG folder = /tmp/tmpBlLjnt
2014-06-04 22:30:35,933 DEBUG Cloning git://github.com/docker-library/nginx into /tmp/tmpBlLjnt
2014-06-04 22:30:35,933 DEBUG Executing "git clone git://github.com/docker-library/nginx ." in /tmp/tmpBlLjnt
Cloning into '.'...
remote: Counting objects: 12748, done.
remote: Compressing objects: 100% (4852/4852), done.
remote: Total 12748 (delta 7668), reused 12743 (delta 7666)
Receiving objects: 100% (12748/12748), 13.85 MiB | 1.40 MiB/s, done.
Resolving deltas: 100% (7668/7668), done.
Checking connectivity... done.
2014-06-04 22:30:47,189 DEBUG Checkout ref:eca68498a6919e324e3fb728f1df014740fcc2ee in /tmp/tmpBlLjnt
2014-06-04 22:30:47,189 DEBUG Executing "git checkout eca68498a6919e324e3fb728f1df014740fcc2ee" in /tmp/tmpBlLjnt
Note: checking out 'eca68498a6919e324e3fb728f1df014740fcc2ee'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b new_branch_name

HEAD is now at eca6849... Add initial Dockerfile
2014-06-04 22:30:47,221 INFO Build start: nginx ('git://github.com/docker-library/nginx', 'eca68498a6919e324e3fb728f1df014740fcc2ee', '.')
2014-06-04 22:31:16,901 INFO Build success: 9d63436f8b39 (nginx:latest)
2014-06-04 22:31:16,906 INFO Build success: 9d63436f8b39 (nginx:1)
2014-06-04 22:31:16,910 INFO Build success: 9d63436f8b39 (nginx:1.7)
2014-06-04 22:31:16,915 INFO Build success: 9d63436f8b39 (nginx:1.7.1)
```
(cc @yosifkit)