Alerting: Fix auth for Nginx in Mimir Backend block (devenv) (#79404)

devenv/docker/blocks/mimir_backend/nginx/nginx.conf.template

@@ -5,9 +5,15 @@ events {}
 http {
   resolver 127.0.0.11 ipv6=off;  
 
+  # Use basic auth user in `X-Scope-OrgID` if the header is not already set.
+  map $http_x_scope_orgid $add_x_scope_orgid {
+    default $http_x_scope_orgid;
+    "" $remote_user;
+  }
+
   server {
     listen 8080;
-    proxy_set_header X-Scope-OrgID $http_x_scope_orgid;
+    proxy_set_header X-Scope-OrgID $add_x_scope_orgid;
 
     location / {
       auth_basic "Mimir Backend";