Update composer Docker tag to v2.5.5

This MR contains the following updates:

Package	Type	Update	Change
composer	final	minor	2.2.9 -> 2.5.5
composer	stage	minor	2.2.9 -> 2.5.5

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docker/wp-config.php

@@ -109,6 +109,12 @@ define('SMTP_FROM', 'noreply@autistici.org');
 define('SMTP_NAME', 'NoBlogs');
 define('SMTP_DEBUG', 0);
 
+/* Set the cookie hash once and for all. Avoids one SQL
+ * lookup on every request. This is just md5(https://noblogs.org/)
+ * to match the value that Wordpress would compute itself.
+ */
+define('COOKIEHASH', '7d12c307721c1cbe9412c639209d72aa');
+
 /**#@+
  * Authentication Unique Keys and Salts.
  *