Use STACK_NAME here

compose.yml

@@ -13,6 +13,7 @@ services:
       - NGINX_WEBSEED_CDN_URI
       - NGINX_WEBSEED_PROXY_ENABLED
       - NGINX_WEBSEED_PROXY_URI
+      - STACK_NAME
     volumes:
       - /etc/timezone:/etc/timezone:ro
       - /etc/localtime:/etc/localtime:ro

nginx.conf.tmpl

@@ -6,7 +6,7 @@ events {
 
 http {
   upstream backend {
-    server app:9000;
+    server {{ env "STACK_NAME" }}_app:9000;
   }
 
   proxy_cache_path /var/cache/nginx/peertube-media-cache levels=1:2 keys_zone=peertube_media_cache:5m max_size=40g inactive=72h use_temp_path=off;