clarify image_vars in containers config is not available at runtime. (#10115)

* clarify in schema comment

* f

Author: emily-shen

packages/wrangler/src/config/environment.ts

@@ -78,7 +78,8 @@ export type ContainerApp = {
 	image_build_context?: string;
 
 	/**
-	 * Image variables to be passed along the image at build time.
+	 * Image variables available to the image at build-time only.
+	 * For runtime env vars, refer to https://developers.cloudflare.com/containers/examples/env-vars-and-secrets/
 	 * @optional
 	 */
 	image_vars?: Record<string, string>;