Bump node from 20-bullseye-slim to 21-bullseye-slim (#40)

Bumps node from 20-bullseye-slim to 21-bullseye-slim.

---
updated-dependencies:
- dependency-name: node
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Matthias Pfeil <matthias.pfeil@wwu.de>

Author: dependabot[bot]

Dockerfile

@@ -2,7 +2,7 @@
 # https://snyk.io/blog/10-best-practices-to-containerize-nodejs-web-applications-with-docker/
 
 # base node image
-FROM node:20-bullseye-slim as base
+FROM node:21-bullseye-slim as base
 
 # set for base and all layer that inherit from it
 # ENV NODE_ENV production