Use a map for passing x-forwarded-proto along

core/nginx/conf/nginx.conf

@@ -20,6 +20,12 @@ http {
     absolute_redirect off;
     resolver {{ RESOLVER }} valid=30s;
 
+    # Header maps
+    map $http_x_forwarded_proto $proxy_x_forwarded_proto {
+      default $http_x_forwarded_proto;
+      ''      $scheme;
+    }
+
     # Main HTTP server
     server {
       # Variables for proxifying

core/nginx/conf/proxy.conf

@@ -2,4 +2,4 @@
 proxy_set_header Host $host;
 proxy_set_header X-Real-IP $remote_addr;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-proxy_set_header X-Forwarded-Proto $scheme;
+proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;