feat(infra): configure imgproxy to use next.js rewrite proxy

- Added /_img/ rewrite rule in next.config.mjs to proxy image requests to IMGPROXY_URL
- Updated lib/imgproxy.ts to use local /_img path instead of exposed public URL
- Replaced NEXT_PUBLIC_IMGPROXY_URL (build-time) with IMGPROXY_URL (runtime)
- Updated Dockerfile and docker-compose.yml to strip unused build args

next.config.mjs

@@ -332,11 +332,20 @@ const nextConfig = {
 
     const directusUrl = process.env.INTERNAL_DIRECTUS_URL || process.env.DIRECTUS_URL || 'https://cms.klz-cables.com';
 
+    let imgproxyUrl = process.env.IMGPROXY_URL || 'https://img.infra.mintel.me';
+    if (!imgproxyUrl.startsWith('http')) {
+      imgproxyUrl = `https://${imgproxyUrl}`;
+    }
+
     return [
       {
         source: '/cms/:path*',
         destination: `${directusUrl}/:path*`,
       },
+      {
+        source: '/_img/:path*',
+        destination: `${imgproxyUrl}/:path*`,
+      },
     ];
   },
 };