nginx cause ERR_TOO_MANY_REDIRECTS when using variable set $upstream

Question

I do have a very odd phenomenon when configuring nginx as reverse proxy. While the following works just fine

location / {
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_pass https://host:8443/;
    proxy_ssl_verify off;
}

The next lines of code causing redirection problems (and browser response with ERR_TOO_MANY_REDIRECTS)

location / {
    resolver 127.0.0.11;
    set $us https://buerger:8443/;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_pass $us;
    proxy_ssl_verify off;
}

The reason why I was trying to setup that way using a variable as upstream, is to silently continue the startup of nginx, even when the host is unavailable at startup.

Im using nginx 1.21.6 as docker container