Updated nginx config to provide correct X-Forwared-Proto header

web/mattermost

@@ -1,3 +1,8 @@
+map $http_x_forwarded_proto $proxy_x_forwarded_proto {
+  default $http_x_forwarded_proto;
+  ''      $scheme;
+}
+
 server {
     listen 80;
 
@@ -10,7 +15,7 @@ server {
         proxy_set_header Host $http_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;
         proxy_set_header X-Frame-Options SAMEORIGIN;
         proxy_pass http://app:APP_PORT;
     }