fix: change upstream for multiple sites

roles/nginx/templates/vhosts/garage.mateu.be.conf.j2

@@ -2,7 +2,7 @@ server {
 {% include './templates/header.conf.j2' %}
 
 	location / {
-		proxy_pass http://s3_backend;
+		proxy_pass http://s3_backend_{{ item.split('.')|join('_') }};
 		proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 		proxy_set_header Host $host;
 		# Disable buffering to a temporary file.
@@ -10,7 +10,7 @@ server {
 	}
 }
 
-upstream s3_backend {
+upstream s3_backend_{{ item.split('.')|join('_') }} {
 	# If you have a garage instance locally.
 	server [::1]:3900;
 }