Не запускается flower в nginx docker

Когда я открываю страницу http://192.168.99.100/flower/ сервер дает ответ:

502 Bad Gateway

docker-compose

    flower:
        build: .
        container_name: flower
        command: flower -A site --url-prefix = flower
        volumes:
            - .:/usr/src/site/
        ports:
            - "5555:5555"

nginx.conf

server {

    listen 80;
    server_name 192.168.99.100;

    location /flower {
        rewrite ^/flower/(.*)$ /$1 break;
        proxy_pass http://192.168.99.100:5555; 
        proxy_redirect off;
        proxy_set_header Host $host;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        proxy_http_version 1.1;     
    }
    
}

в консоле

[error] 21#21: *13 upstream prematurely closed connection while reading response header from upstream, client: 192.168.99.1, server: 192.168.99.100, request: "GET /flower/ HTTP/1.1", upstream: "http://192.168.99.100:5555/", host: "192.168.99.100"

Из-за чего flower не запускается??