for review by BR

@rakitzis Could you approve this fix?
This splits a DOCKER_HOST of the form: "tcp://127.0.0.1:69". For what it's worth, it won't work if DOCKER_HOST is set to "127.0.0.1:69". That needs an extra check. I suggest:

```
if len(parts) == 1 { // proto defaults to tcp
    parts = append([]string{"tcp"}, parts[0])
}
```