⬆️(mail) upgrade Node image for mails-install to align with the frontend

Node 18 is now legacy, and running `make mails-install` was failing
with a "bad engine" error against the current image.

Bump the Node image used for `mails-install` to a more recent
version, aligned with the one used by the frontend, so the make
target runs cleanly again.

Note: the Alpine variant cannot be used here, as `bash` is required
to build the mail templates.

compose.yml

@@ -180,7 +180,7 @@ services:
     working_dir: /app
 
   node:
-    image: node:18
+    image: node:22
     user: "${DOCKER_USER:-1000}"
     environment:
       HOME: /tmp