Update `Dockerfile` and docs

[zugazagoitia]

• Simplified Dockerfile and reduced image size by 60%+
• Simplified Docker docs
• Use github-pages gem recommended ruby version

[github-actions[bot]]

❌ Delete Preview for javalin.io failed.

Name	Link
🔨 Latest commit	09fef4f310a3644bc21800fb6222b7a55b1fe0c5
🔍 Latest deploy log	https://github.com/javalin/javalin.github.io/actions/runs/5824063968

[tipsy]

Thanks!