fix: use explicit node version for docusaurus build

docs/Dockerfile

@@ -2,7 +2,7 @@
 
 # Stage 1: Base image.
 ## Start with a base image containing NodeJS so we can build Docusaurus.
-FROM node:lts as base
+FROM node:18-alpine3.18 as base
 ## Disable colour output from yarn to make logs easier to read.
 ENV FORCE_COLOR=0
 ## Enable corepack.