add Dockerfile for Docker Trusted Build

This is mostly there. It needs a couple things, yet, namely:

 - [ ] Who's the `MAINTAINER`?
 - [x] I'm able to run this, and everything appears to be fine, but Mesos never says it's registered. I'm running it like so: `docker run -d -p 8080:8080 mesosphere/marathon --master zk://192.168.1.104:2181/mesos --zk_hosts 192.168.1.104:2181 --hostname 192.168.1.104:8080` (where `192.168.1.104` is the IP of the parent Vagrant host, which is also running Mesos and zookeeper)
 - [x] install to `/opt` instead of `/etc`

(this PR in reference to #76)