message: docs/conf: add env var substitution docs
diff: @@ -161,3 +161,19 @@ Example `my-secrets.json` file:
  }
  }
  ```
  +
  +## Environment Variable Substitution
  +
  +Configuration files support environment variable substitution via a `${MY_VAR}`
  +syntax. For example:
  +
  +```yaml
  +app:
  + baseUrl: https://${HOST}
  +```
  +
  +Note that all environment variables must be available, or the entire
  +configuration value will evaluate to `undefined`.
  +
  +The substitution syntax can be escaped using `$${...}`, which will be resolved
  +as `${...}`.
  