refactor: use environment variable for GHOST_API_KEY

Replace hardcoded GHOST_API_KEY with process.env.GHOST_API_KEY in 
API routes to enhance security. Update Docker commands in the CI 
workflow to pass the GHOST_API_KEY as an environment variable. 
Add GHOST_API_KEY to the Next.js configuration for consistent 
access across the application.

next.config.ts

@@ -1,7 +1,9 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  env: {
+    GHOST_API_KEY: process.env.GHOST_API_KEY,
+  },
 };
 
 export default nextConfig;