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

Dockerfile

@@ -5,7 +5,6 @@ WORKDIR /app
 # Arguments for build-time configuration
 ARG NEXT_PUBLIC_BASE_URL
 ARG NEXT_PUBLIC_TARGET
-ARG NEXT_PUBLIC_IMGPROXY_URL
 ARG DIRECTUS_URL
 ARG UMAMI_WEBSITE_ID
 ARG UMAMI_API_ENDPOINT
@@ -14,7 +13,6 @@ ARG NPM_TOKEN
 # Environment variables for Next.js build
 ENV NEXT_PUBLIC_BASE_URL=$NEXT_PUBLIC_BASE_URL
 ENV NEXT_PUBLIC_TARGET=$NEXT_PUBLIC_TARGET
-ENV NEXT_PUBLIC_IMGPROXY_URL=$NEXT_PUBLIC_IMGPROXY_URL
 ENV DIRECTUS_URL=$DIRECTUS_URL
 ENV UMAMI_WEBSITE_ID=$UMAMI_WEBSITE_ID
 ENV UMAMI_API_ENDPOINT=$UMAMI_API_ENDPOINT