fix(nginx): bake nginx.conf into the image instead of bind-mounting

User reported persistent 502 with upstream "frontend:3000" after the
previous fix that changed the upstream to "frontend:80". Symptom is
a stale conf still being served by the nginx container - the host
volume mount was keeping an old file in play (cached image, missing
git pull, or the conf simply not being re-read).

Make this class of bug impossible: ship the conf inside the nginx
service's image. A fresh build now guarantees the conf in the
container matches the conf in the repo.

- nginx/Dockerfile added (FROM nginx:alpine + COPY nginx.conf)
- docker-compose nginx service: image -> build ./nginx; remove
  the conf bind mount entirely.

Deploy:  git pull && docker compose build nginx frontend && docker compose up -d --force-recreate

docker-compose.yml

@@ -31,12 +31,10 @@ services:
       - mc-filter
 
   nginx:
-    image: nginx:alpine
+    build: ./nginx
     container_name: mc-filter-nginx
     ports:
       - "8080:80"   # 대시보드 접근 포트 (외부 포트포워딩 금지 권장)
-    volumes:
-      - ./nginx/nginx.conf:/etc/nginx/nginx.conf:ro
     depends_on:
       - api
       - frontend