add with dockerfile

crates/dagger-sdk/examples/existing-dockerfile/app/README.md

@@ -0,0 +1,17 @@
+# React Build
+
+This is based on the [Getting Started guide for Nodejs](https://docs.dagger.io/sdk/nodejs/783645/get-started#step-5-test-against-multiple-nodejs-versions)
+
+A simple react app is created with `create-react-app` which is built and tested by `build.js` or `build.ts`.
+
+Run:
+
+`npm install`
+
+and then:
+
+`node --loader ts-node/esm ./build.ts`
+
+or
+
+`node ./build.js`