Fix: Pass FASTAPI_URL as build arg for Next.js rewrites

Next.js rewrites are evaluated at build time, not runtime.
Without FASTAPI_URL set during build, the rewrite destination
defaults to localhost:8000 which fails in Docker.

- Add FASTAPI_URL build arg to nextjs-app/Dockerfile
- Pass build arg in docker-compose.yml
- Pass build arg in Gitea Actions workflow

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -54,6 +54,8 @@ services:
     build:
       context: ./nextjs-app
       dockerfile: Dockerfile
+      args:
+        FASTAPI_URL: http://backend:80/api
     container_name: schoolcompare_nextjs
     ports:
       - "3000:3000"