yeah, nginx config for lemmy is not very straighforward. you need to mimic this:

worker_processes  1;
events {
    worker_connections  1024;
}
http {
    upstream lemmy {
        server "lemmy:8536";
    }
    upstream lemmy-ui {
        server "lemmy-ui:1234";
    }
    server {
      listen       1236;
      server_name  localhost;

      # frontend
      location / {
        set $proxpass "http://lemmy-ui";
        if ($http_accept = "application/activity+json") {
          set $proxpass "http://lemmy";
        }
        if ($http_accept = "application/ldr+json; profile=\"https://www.w3.org/ns/activitystreams\"") {
          set $proxpass "http://lemmy";
        }
        if ($request_method = POST) {
          set $proxpass "http://lemmy";
        }
        proxy_pass $proxpass;

        rewrite ^(.+)/+$ $1 permanent;

        # Send actual client IP upstream
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      }

      # backend
      location ~ ^/(api|pictrs|feeds|nodeinfo|.well-known) {
        proxy_pass "http://lemmy";
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";

        # Add IP forwarding headers
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      }
    }
}

also - can you check if all containers are running? just do docker-compose ps in the lemmy dir.