Adding step to build the app with docker in existing projects

[sespinosa]

Added this paragraph to examples/with-docker/README.md:

In existing projects

To add support for docker to an existing project, just copy the Dockerfile into the root of the project and add the following to the next.config.js file:

// next.config.js
module.exports = {
  // ... rest of the configuration.
  experimental: {
    outputStandalone: true,
  }
}

This will build the project as a standalone app inside the Docker image.