Docker: bump assetbuilder image to node:21 (closes #203)

Change-Id: I6c770383172fa2ef5f339246add7c483dbb109ae
diff --git a/Dockerfile b/Dockerfile
index 3f136c5..02115f1 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,5 +1,5 @@
 # Build assets in builder image
-FROM node:20 AS assetbuilder
+FROM node:21 AS assetbuilder
 
 WORKDIR '/app'