Add a note about modules missing from dependencies (#2298)

Author: clouvet

js/frameworks/nextjs.html.markerb

@@ -274,3 +274,14 @@ Follow the instructions in the message, and then redeploy:
 npm i sharp
 fly deploy
 ```
+
+### Cannot find module
+
+Your app doesn't start and you see an error like the following in your logs:
+```
+`Cannot find module 'typescript'`
+```
+
+Verify that the missing module appears in `package.json`'s `dependencies`, not just `devDependencies`. If you're using a Dockerfile generated by `dockerfile-node`, your image will be optimized for production and omit development dependencies by default.
+
+If the module is missing, add it to `dependencies` and re-deploy.