Pass DOMAIN build arg to Dockerfile build stage

Without ARG/ENV, process.env.DOMAIN was undefined during npm run build,
causing llms.txt internal links to use localhost:4321 instead of the
production domain.

docs/Dockerfile

@@ -15,6 +15,8 @@ CMD ["npm", "run", "dev", "--", "--host", "0.0.0.0"]
 FROM base AS build
 RUN npm ci
 COPY . .
+ARG DOMAIN=localhost:4321
+ENV DOMAIN=${DOMAIN}
 ENV ASTRO_TELEMETRY_DISABLED=1
 RUN --mount=type=cache,target=/app/.astro \
     npm run build