Add option to provide custom Dockerfile

[madAndroid]

First of all, thanks - great project! we were doing a lot of these steps ourselves, and having these built into a gem will save us loads of time.

Would you consider having an option to provide a custom Dockerfile? we install golang in our Dockerfile in our previous build process, since this is a dependency for some of the sensu plugins we install. Having this in the Dockerfile means being able to use all the docker caching goodness, and allows us to track any changes we need.

I'm currently editing the fpm-dockery-0.1.8/docker/Dockerfile.centos7 file and rebuilding the image in order to accomplish this, but it would be great to be able to set this as an arg to the command.