Docker requires DOCKER_HOST env to use format localhost:3000 without the scheme

@bfulton @mattheworiordan

#172 with fixed cane error
```
$ DOCKER_HOST=localhost docker images
2014/08/06 14:11:44 Invalid bind address format: localhost
$ DOCKER_HOST=localhost:2375 docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
<none>              <none>              912206c2c2d6        46 hours ago        1.186 GB
```

If no port is provided, docker fails.