Update node Docker tag to v21

| datasource | package | from | to |
| ---------- | ------- | ---- | -- |
| docker     | node    | 20   | 21 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.docker/php/Dockerfile

@@ -1,7 +1,7 @@
 # DOCKER-VERSION        1.3.2
 
 # Build UI assets
-FROM node:20-alpine as assets
+FROM node:21-alpine as assets
 RUN apk add git
 RUN npm install --location=global gulp-cli
 RUN mkdir -p /work