Nginx 代理过程中的字符串插值

我试图为docker容器创建一个nginx反向代理，出于某种原因，当我尝试插入包含服务主机名的变量并尝试访问

得不到/

这很有效

server {

  resolver 127.0.0.11 valid=300s;
  resolver_timeout 60s;

  listen 443 default_server ssl;
  server_name localhost;

  ssl_certificate /etc/ssl/certs/nginx-selfsigned.crt;
  ssl_certificate_key /etc/ssl/private/nginx-selfsigned.key;

  location /services/my-service/ {
    set $service_url my-service;
    proxy_pass http://my-service/;
    proxy_set_header Host $host;
    proxy_redirect http:// https://;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
  }
}

但事实并非如此

server {

  resolver 127.0.0.11 valid=300s;
  resolver_timeout 60s;

  listen 443 default_server ssl;
  server_name localhost;

  ssl_certificate /etc/ssl/certs/nginx-selfsigned.crt;
  ssl_certificate_key /etc/ssl/private/nginx-selfsigned.key;

  location /services/my-service/ {
    set $service_url my-service;
    proxy_pass http://$service_url/;
    proxy_set_header Host $host;
    proxy_redirect http:// https://;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
  }
}