Temporarily use TCP for docker

Temporary workaround for #163.

We'd be waiting on [this PR](https://github.com/dotcloud/docker/pull/1485) from the docker team for a better solution but in the mean time this should allow new users to continue to use dokku. 

After reviewing documentation I could not find a way to specify default options for the `docker` CLI client resulting in the off looking `-H tcp://`s littered around. (If anyone else sees a way please let me know!)

I recommend that this commit be reverted and `git` be added to the `docker` group after [docker#1485](https://github.com/dotcloud/docker/pull/1485) is merged and released to launchpad.