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

lib/imgproxy.ts

@@ -30,10 +30,8 @@ function encodeBase64(str: string): string {
 }
 
 export function getImgproxyUrl(src: string, options: ImgproxyOptions = {}): string {
-  const baseUrl = process.env.NEXT_PUBLIC_IMGPROXY_URL || 'https://img.infra.mintel.me';
-
-  // If no imgproxy URL is configured, return the source as is
-  if (!baseUrl) return src;
+  // Use local proxy path which is rewritten in next.config.mjs
+  const baseUrl = '/_img';
 
   // Handle local paths or relative URLs
   let absoluteSrc = src;