Mention there's no Docker deamon running on macOS hosted agents

I found it a little surprising, while attempting to build a Docker image on a macOS hosted agent, that the Docker daemon wasn't there:

```
docker:index:Image f84b9ef5-container error: Docker native provider returned an unexpected error from Configure: failed to connect to any docker daemon
```

It also wasn't clear to me what the best course of action was.

We should mention this in the docs so when users go Googling for it, we have something for them that explains it, and ideally a recommendation for how to fix it.