fix(ci): rename Dockerfile.nginx to Dockerfile.proxy to match workflow matrix

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/docker/Dockerfile.proxy

@@ -0,0 +1,7 @@
+FROM nginx:alpine
+
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80
+
+CMD ["nginx", "-g", "daemon off;"]