Fix ENV name typo

Author: pouwerkerk

content/manuals/build/building/variables.md

@@ -158,7 +158,7 @@ COPY . .
 CMD ["node", "app.js"]
 ```
 
-With this Dockerfile, you can use `--build-arg` to override the default value of `ENV`:
+With this Dockerfile, you can use `--build-arg` to override the default value of `NODE_ENV`:
 
 ```console
 $ docker build --build-arg NODE_ENV=development .