docs: add note that .env is not read in production (#4714)

docs/content/2.guide/2.features/10.runtime-config.md

@@ -33,10 +33,15 @@ console.log(runtimeConfig.public.apiBase)
 ### Environment Variables
 
 The most common way to provide configuration is by using [Environment Variables](https://medium.com/chingu/an-introduction-to-environment-variables-and-how-to-use-them-f602f66d15fa).
+
+::alert{type=info}
 Nuxt CLI has built-in [dotenv](https://github.com/motdotla/dotenv) support.
 
 In addition to any process environment variables, if you have a `.env` file in your project root directory, it will be automatically loaded into `process.env` and accessible within your `nuxt.config` file and modules.
 
+However, **after your project is built**, you are responsible for setting environment variables when you run the server - your `.env` file will not be read at this point.
+::
+
 Runtime config values are automatically replaced by matching environment variables at runtime. For this to work, you _must_ have a fallback value (which can just be an empty string) defined in your `nuxt.config`.
 
 **Example:**