Flask + Nginx + Docker-compose 遇到 502 Bad Gateway 问题

请教一下，我用 docker-compose 部署 Flask + Nginx + Docker-compose + gunicorn 应用，本地访问 0.0.0.0:5000 可以访问，访问 0.0.0.0:80 显示 502 bad gate，但 0.0.0.0:80/static/… 静态文件能成功访问，不知道是配置文件哪个地方配错，有没有什么建议？非常感谢

docker-compose.yml

version: '3'

services:
  webapp:
    build: .
    # command: bash ./start_server.sh
    # env_file: .env
    volumes:
      - ./webapp:/iodock/
    ports:
      - "5000:5000"  # web 容器内的 5000 端口映射到主机的 5000 端口
    depends_on:
      - nginx

  nginx:
    restart: always
    image: nginx:stable
    volumes:
      - ./webconf/nginx/conf.d/:/etc/nginx/conf.d/
      - ./webconf/nginx/log/:/var/log/nginx/
      - ./webconf/nginx/cert/:/opt/cert/
      - ./webapp/app/static/:/opt/static/
    ports:
      - "80:80"
      - "443:443"

Dockerfile

# --------------------------
# Docker file
# --------------------------
FROM python:3.6

# Set system language
ENV LC_ALL C.UTF-8
ENV LANG C.UTF-8

# install python requirement
COPY ./webapp/requirements.txt /tmp/requirements.txt
RUN pip3 install -i https://pypi.tuna.tsinghua.edu.cn/simple --upgrade pip
RUN pip3 install --no-cache-dir gunicorn
RUN pip3 install -i https://pypi.tuna.tsinghua.edu.cn/simple -r /tmp/requirements.txt

# Create root folder docker_web_app
COPY ./webapp /docker_web_app/iodock/
WORKDIR /iodock/

# ENV PORT 5000
EXPOSE 5000

CMD ["/usr/local/bin/gunicorn", "-w", "2", "-b", ":5000", "manage:app"]

nginx.conf

server {
    listen  80;
    server_name     localhost;                           # 公网地址
    access_log      /var/log/nginx/nginx_access.log;
    error_log       /var/log/nginx/nginx_error.log;

    location / {
        proxy_pass         http://localhost:5000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }

    location /static {
        alias /opt/static;
        proxy_set_header Host $host;
        # proxy_cache mycache;
        # expires 30d;
    }
}

nginx_error.log

2019/04/21 14:36:18 [error] 7#7: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:5000/", host: "0.0.0.0"
2019/04/21 14:36:18 [error] 7#7: *1 connect() failed (111: Connection refused) while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET / HTTP/1.1", upstream: "http://127.0.0.1:5000/", host: "0.0.0.0"
2019/04/21 14:36:19 [error] 7#7: *1 no live upstreams while connecting to upstream, client: 172.18.0.1, server: localhost, request: "GET /favicon.ico HTTP/1.1", upstream: "http://localhost/favicon.ico", host: "0.0.0.0", referrer: "http://0.0.0.0/"