fix(ui): update node to v20 (#15717)

### What?
Updated the Docker Node.js image version from `18-alpine` to `20-alpine`
in the `docker-compose.yml` file.

### Why?
Next.js 15 and Payload CMS 3 require a more recent Node.js runtime.
Node.js 18 lacks the global `File` object, which leads to a
`ReferenceError: File is not defined` during Server-Side Rendering
(SSR). Upgrading to Node.js 20+ provides native support for the global
File API, resolving this runtime crash.

### How?
Modified the `image` property under the `payload` service in
`docker-compose.yml` to use `node:20-alpine`.

Co-authored-by: Paul Popus <paul@payloadcms.com>

Author: ab192130

templates/blank/docker-compose.yml

@@ -2,7 +2,7 @@ version: '3'
 
 services:
   payload:
-    image: node:18-alpine
+    image: node:20-alpine
     ports:
       - '3000:3000'
     volumes: