ref(router): optimize Dockerfile for smaller image

Builds nginx 1.6.1 from source, discarding the build tools afterward. Reduces the virtual size from 368MB to 180MB, at the cost of increased time for iterative development. Refs #1825, #1850.

```console
$ # worst-case build times: no local docker layers
$ time docker build -t deis/router:optimize .
Sending build context to Docker daemon 27.65 kB
...
Removing intermediate container 915cac6ddc8e
Successfully built a8f29a230fc8

real	2m29.080s
user	0m0.064s
sys	0m0.058s
$ git checkout master
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.
$ time docker build -t deis/router:master .
Sending build context to Docker daemon 26.62 kB
...
Successfully built 7fed8dc2880c

real	3m27.248s
user	0m0.101s
sys	0m0.076s
$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
deis/router         master              7fed8dc2880c        29 minutes ago      367.9 MB
deis/router         optimize            a8f29a230fc8        34 minutes ago      180.4 MB
ubuntu              12.04               c17f3f519388        12 days ago         106.7 MB
deis/base           latest              589b5b05090d        5 weeks ago         218.2 MB
```