Routing issue with installation in a sub-directory

ref: https://github.com/symfony/symfony/issues/7129

src/Symfony/Bundle/FrameworkBundle/Routing/Router.php

@@ -94,7 +94,7 @@ class Router extends BaseRouter implements WarmableInterface
                 foreach ($route->getRequirements() as $name => $value) {
                      $route->setRequirement($name, $this->resolveString($value));
                 }
-
+                $collection->setPrefix('/'.ltrim($this->resolveString($collection->getPrefix()), '/'));
                 $route->setPattern($this->resolveString($route->getPattern()));
             }
         }

src/Symfony/Component/Routing/RouteCollection.php

@@ -259,6 +259,16 @@ class RouteCollection implements \IteratorAggregate, \Countable
         return $this->prefix;
     }
 
+    /**
+     * Sets the prefix (used to resolve placeholders).
+     *
+     * @param string The prefix
+     */
+    public function setPrefix($prefix)
+    {
+        $this->prefix = $prefix;
+    }
+
     /**
      * Returns an array of resources loaded to build this collection.
      *