UI: Build Dockerfile with Node v14 (#6712)

Node versions >14 throw errors when building and thus do not allow to
build the UI.

Pinning to v14 allows us to build and run the UI Docker container.

Author: wido

ui/Dockerfile

@@ -17,7 +17,7 @@
 
 # Build example: docker build -t <name> .
 
-FROM node:lts-stretch AS build
+FROM node:14-bullseye AS build
 
 MAINTAINER "Apache CloudStack" <dev@cloudstack.apache.org>
 LABEL Description="Apache CloudStack UI; Modern role-base progressive UI for Apache CloudStack"