Adding step to build the app with docker in existing projects

examples/with-docker/README.md

@@ -20,6 +20,21 @@ yarn create next-app --example with-docker nextjs-docker
 
 You can view your images created with `docker images`.
 
+### 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:
+```js
+// 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.
+
 ## Deploying to Google Cloud Run
 
 1. Install the [Google Cloud SDK](https://cloud.google.com/sdk/docs/install) so you can use `gcloud` on the command line.