Add .dockerignore for node_modules & explanation

Author: dmahnkopf

docs/tutorial/image-building-best-practices/index.md

@@ -79,6 +79,21 @@ a change to the `package.json`. Make sense?
     CMD ["node", "/app/src/index.js"]
     ```
 
+1. Create a file named `.dockerignore` in the same folder as the Dockerfile with the following contents.
+
+    ```ignore
+    node_modules
+    ```
+
+    `.dockerignore` files are an easy way to selectively copy only image relevant files.
+    You can read more about this
+    [here](https://docs.docker.com/engine/reference/builder/#dockerignore-file).
+    In this case, the `node_modules` folder should be omitted in the second `COPY` step because otherwise,
+    it would possibly overwrite files which were created by the command in the `RUN` step.
+    For further details on why this is recommended for Node.js applications and other best practices,
+    have a look at their guide on
+    [Dockerizing a Node.js web app](https://nodejs.org/en/docs/guides/nodejs-docker-webapp/).
+
 1. Build a new image using `docker build`.
 
     ```bash