How to use letsencrypt-acme-challenge.conf in Nginx

Because I always forget, if you're using certbot to create certs for your Nginx server, you'll need to it up so it works on HTTP as well as HTTPS. But once you're done, you're going to want all HTTP traffic to redirect to HTTPS. The correct syntax is:

server {
    server_name mydomain.example.com;
    include /etc/nginx/snippets/letsencrypt-acme-challenge.conf;
    location / {
      return 301 https://mydomain.example.com$request_uri;
    }
}

And that letsencrypt-acme-challenge.conf looks like this (code comments stripped):

location ^~ /.well-known/acme-challenge/ {
    default_type "text/plain";
    root         /var/www/html;
    break;
}
location = /.well-known/acme-challenge/ {
    return 404;
}

This way, a GET request for http://mydomain.example.com/.well-known/acme-challenge/test.html will be 200 OK if there's a file called /var/www/html/.well-known/acme-challenge/test.html. And http://mydomain.example.com/.well-known/acme-challenge/does-not-exist.html will 404 Not Found.

But all and any other GET request will redirect. E.g. http://mydomain.example.com/whatever -- 301 Moved Permanently --> https://mydomain.example.com/whatever.