Add slim Docker image temple to readme examples

[desert-dev]

Describe your issue
Currently people might miss on opportunity to optimize Docker image size, if needed.
This example will provide both awareness of that option and and a template, to quickly adjust to developers needs.

[zugazagoitia]

Hi, we already have a docker tutorial in the website (javalin/javalin.github.io):

https://javalin.io/tutorials/docker

That might be a better place to add it, and if you want, a better explanation that we currently provide.