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

nginx/Dockerfile

@@ -0,0 +1,6 @@
+# nginx.conf 를 image 에 굽는다.
+# (volume mount 로 운영하다 보면 호스트 측 conf 가 stale 일 때 영문도 모르고
+#  502 가 나는 경우가 있어 image 안에 직접 포함시킨다.)
+FROM nginx:alpine
+COPY nginx.conf /etc/nginx/nginx.conf
+EXPOSE 80