build: use `node:18.20-alpine3.19 in Docker

Chromium 128.0.6613.84-r0 (used in Alpine 3.20) seems to be having
issues with puppeteer/mermaid-cli, so we can't use it yet.

See: 294af9f
Fixes: #762

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:alpine
+FROM node:18.20-alpine3.19
 
 ENV CHROME_BIN="/usr/bin/chromium-browser" \
     PUPPETEER_SKIP_DOWNLOAD="true"