Created a config prost-processor which adds the base path on every applicable configuration

module/Core/src/Config/BasePathPrefixer.php

@@ -0,0 +1,39 @@
+<?php
+declare(strict_types=1);
+
+namespace Shlinkio\Shlink\Core\Config;
+
+use function Functional\map;
+
+class BasePathPrefixer
+{
+    public function __invoke(array $config): array
+    {
+        $basePath = $config['router']['base_path'] ?? '';
+        $config['routes'] = $this->prefixRoutesWithBasePath($config, $basePath);
+        $config['middleware_pipeline'] = $this->prefixMiddlewarePathsWithBasePath($config, $basePath);
+        $config['url_shortener']['domain']['hostname'] .= $basePath;
+
+        return $config;
+    }
+
+    private function prefixRoutesWithBasePath(array $config, string $basePath): array
+    {
+        return map($config['routes'] ?? [], function (array $route) use ($basePath) {
+            $route['path'] = $basePath . $route['path'];
+            return $route;
+        });
+    }
+
+    private function prefixMiddlewarePathsWithBasePath(array $config, string $basePath): array
+    {
+        return map($config['middleware_pipeline'] ?? [], function (array $middleware) use ($basePath) {
+            if (! isset($middleware['path'])) {
+                return $middleware;
+            }
+
+            $middleware['path'] = $basePath . $middleware['path'];
+            return $middleware;
+        });
+    }
+}