Update node Docker tag to v21

| datasource | package | from   | to     |
| ---------- | ------- | ------ | ------ |
| docker     | node    | 20.6.1 | 21.6.0 |


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

Author: renovate[bot]

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:20.6.1-alpine as compiler
+FROM node:21.6.0-alpine as compiler
 
 RUN mkdir -p /usr/local/source
 WORKDIR /usr/local/source
@@ -10,7 +10,7 @@ COPY ./src ./src
 RUN npm run build
 
 
-FROM node:20.6.1-alpine
+FROM node:21.6.0-alpine
 LABEL "repository"="http://github.com/laminas/laminas-ci-matrix-action"
 LABEL "homepage"="http://github.com/laminas/laminas-ci-matrix-action"
 LABEL "maintainer"="https://github.com/laminas/technical-steering-committee/"