resolver does not work

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Hello,
I have a nginx proxy in front of systemd-nspawn containers.
The IP's of the containers are dynamic.

When I start nginx *after* the  containers it works.
When the IP of the container is changed while nginx is running i get a 
"Bad Gateway" error.

The config looks like:

server {
     server_name foobar.example.com
...
    location / {
       resolver 127.0.0.53 valid=10s;
       ...
       proxy_pass http://container;
    }
}

nginx is 1.1.18 so it should work as documented in 
http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver
The workaround there 
https://stackoverflow.com/questions/42720618/docker-nginx-stopped-emerg-11-host-not-found-in-upstream/52319161#52319161 
doesn't work.

I have also try to config a upstream backend and the resolver in the 
server part or in the http part.
The errors are: "upstream timed out" or "container could not be resolved 
(3: Host not found)"

Whats wrong there?
Best Regards