Fix grep regex escaping for extended regex (ERE)

Replace preg_quote() with proper ERE escaping since grep -E uses
extended regex syntax, not PHP/PCRE. This ensures special characters
in registry URLs (dots, etc.) are properly escaped.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/Actions/Server/CleanupDocker.php

@@ -89,8 +89,9 @@ private function buildImagePruneCommand($applicationImageRepos): string
         } else {
             // Build grep pattern to exclude application image repositories
             $excludePatterns = $applicationImageRepos->map(function ($repo) {
-                // Escape special characters for grep basic regex
-                return preg_quote($repo, '/');
+                // Escape special characters for grep extended regex (ERE)
+                // ERE special chars: . \ + * ? [ ^ ] $ ( ) { } |
+                return preg_replace('/([.\\\\+*?\[\]^$(){}|])/', '\\\\$1', $repo);
             })->implode('|');
 
             // Delete unused images that: