Override serverUrl in wasp deploy fly deploy command (#2234)

waspc/packages/deploy/src/providers/fly/deploy/deploy.ts

@@ -103,8 +103,13 @@ async function deployClient(deploymentInfo: DeploymentInfo<DeployOptions>, { bui
 	copyProjectClientTomlLocally(deploymentInfo.tomlFilePaths);
 
 	waspSays('Building web client for production...');
+	waspSays('If you configured a custom domain for the server, you should run the command with an env variable: REACT_APP_API_URL=https://serverUrl.com wasp deploy fly deploy');
+
+	const serverUrl = process.env.REACT_APP_API_URL
+		? process.env.REACT_APP_API_URL
+		: deploymentInfo.serverUrl;
 	await $`npm install`;
-	await $`REACT_APP_API_URL=${deploymentInfo.serverUrl} npm run build`;
+	await $`REACT_APP_API_URL=${serverUrl} npm run build`;
 
 	// Creates the necessary Dockerfile for deploying static websites to Fly.io.
 	// Adds dummy .dockerignore to supress CLI question.