I currently have the configuration:

proxy_cache_path /tmp/nginx levels=1:2 keys_zone=my_zone:180m inactive=60m max_size=700m;
proxy_cache_valid 200 302 10m;
proxy_cache_valid 404 1m;
proxy_cache_use_stale  error timeout invalid_header updating http_500 http_502 http_503 http_504;

...

server_name example.com
location / {
    proxy_cache my_zone;
    proxy_cache_key "$scheme://$host$uri";
    proxy_cache_valid 200 301 302 100d;
    proxy_cache_bypass  $http_cache_control;
    proxy_cache_use_stale  error timeout invalid_header updating http_500 http_502 http_503 http_504;
    add_header X-Proxy-Cache $upstream_cache_status;

    include proxy_params;
    proxy_pass http://127.0.0.1:8000;
}

But when accessing the server I always seem get x-proxy-cache: MISS:

$ curl -I https://example.com

How come?