Automatically build image if Dockerfile is present

**Feature Idea - Looking for votes and/or user input to gauge traction.**

If a `Dockerfile` is present in the current directory, automatically build the image and run the container when the host is started ->

On `func host start` , run:

1.
``` bash
> docker build . -t
```

2.
``` bash
> docker run -p 80:80
```

Similarly, during publish, automatically build and publish the image to the given registry and use it to start a Linux(Docker) dedicated app.

``` bash
> azure functionapp publish {function_app_name} --registry
```