use skynet server api env variable to rewrite proxy response on /tus endpoint

docker/nginx/conf.d/client.conf

@@ -334,7 +334,7 @@ server {
 		proxy_pass http://siad;
 
 		# rewrite tus headers to use correct uri
-		proxy_redirect https://siad/ https://$domain.$tld/;
+		proxy_redirect https://siad $SKYNET_SERVER_API;
 
 		# extract skylink from base64 encoded upload metadata and assign to a proper header
 		header_filter_by_lua_block {

docker/nginx/nginx.conf

@@ -25,8 +25,10 @@ worker_processes  1;
 
 #pid        logs/nginx.pid;
 
-env SKYNET_PORTAL_API; # declare env variable to use it in config
-env ACCOUNTS_ENABLED; # declare env variable to use it in config
+# declare env variables to use it in config
+env SKYNET_PORTAL_API;
+env SKYNET_SERVER_API;
+env ACCOUNTS_ENABLED;
 
 events {
     worker_connections  1024;