use variable for fastcgi_pass to force resolver usage

.docker/web-nginx/nginx.conf

@@ -18,10 +18,6 @@ http {
 
 	resolver ${RESOLVER} valid=5s;
 
-	upstream app {
-		server ${APP_UPSTREAM}:9000;
-	}
-
 	server {
 		listen 80;
 		listen [::]:80;
@@ -53,7 +49,9 @@ http {
 			fastcgi_index index.php;
 			include fastcgi.conf;
 
-			fastcgi_pass app;
+			set $backend "${APP_UPSTREAM}:9000";
+
+			fastcgi_pass $backend;
 		}
 
 		location / {