Add recipe for Docker image

### What do you want and why?

There should be an easy way to package Blitz applications in a Docker image.
Therefore, It'd be great to have some kind of well-thought-out, fully working Dockerfile available to users.

### Possible implementation(s)

The Dockerfile could either be part of the `blitz new` template or be generated using [installers](https://github.com/blitz-js/blitz/issues/242).

### Additional context

Some more thoughts:

- The Dockerfile needs to reside in the root folder. It's a convention that enables things like GitLab's AutoDevOps to work "magically".
- Obviously, we'd need documentation for this.