Problem with nginx reverse proxy and Tailscale at boot time

Question

I'm using Tailscale and an nginx reverse proxy to access my Home Assistant server. The following nginx.conf snippet works fine:

location / {
    proxy_pass http://home-hassio:8123;

    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header X-Real-IP $remote_addr;
}

However, I run into a problem at boot time - since Nginx loads before Tailscale, it fails because it can't get the IP address of the home-hassio host. Based on various sources, I introduced a variable to evaluate the domain IP at runtime. However, the following doesn't work at all.

location / {
    resolver 100.100.100.100;
    resolver_timeout 1s;
    set $upstream http://home-hassio:8123;
    proxy_pass $upstream;

    proxy_http_version 1.1;
    proxy_set_header upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header X-Real-IP $remote_addr;
}

But now, I'm getting 502 Bad Gateway error. What am I doing wrong?