This ensures that the value of NODE_ENV is made available to the CMD
context. This ensures commands such as `npm start` or `node index.js`
are using the correct NODE_ENV.
See comments in
https://github.com/docker-library/official-images/pull/2156
It's currently impossible to prevent npm installing devDependencies when using the node onbuild images. Adding a NODE_ENV ARG enables `docker build --build-arg NODE_ENV=production .`, which is the equivalent of `npm install --production`. Providing the `--build-arg` is optional, so this is backwards-compatible.
Peter Petrov
Andrew Euell
Christopher Horrell
Mark Woodbridge
Laurent Goderre