Update node.js to v14 in lesspass-site Dockerfile

The node version on CI for it in .github/workflows/containers-test.yml
is v14, and it looks like it works well with node.js v14

This also syncs the nodejs version as the lesspass-website package is
also using node.js v14

packages/lesspass-site/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:12-alpine AS builder
+FROM node:14-alpine AS builder
 LABEL maintainer="LessPass <contact@lesspass.com>"
 LABEL name="LessPass Frontend"
 WORKDIR /opt/frontend