misc: Pin Node version to v20

Avoid the stack being automatically upgraded to a new Node version, when
a different LTS is released.

docker/Dockerfile

@@ -1,6 +1,6 @@
 ARG ALPINE_VERSION=3.20
 ARG NGINX_VERSION=1.27.1
-ARG NODE_VERSION=lts
+ARG NODE_VERSION=20.16
 ARG PYTHON_VERSION=3.12
 
 # Build frontend