fix: Dockerfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-DEBIAN12-SYSTEMD-6277507
- https://snyk.io/vuln/SNYK-DEBIAN12-SYSTEMD-6277507
- https://snyk.io/vuln/SNYK-DEBIAN12-ZLIB-6008963
- https://snyk.io/vuln/SNYK-DEBIAN12-GNUTLS28-6474581
- https://snyk.io/vuln/SNYK-DEBIAN12-GNUTLS28-6474586

Dockerfile

@@ -4,7 +4,7 @@
 #
 # Author: muxator
 
-FROM node:lts-slim
+FROM node:20.15.0-slim
 LABEL maintainer="Etherpad team, https://github.com/ether/etherpad-lite"
 
 ARG TIMEZONE=