Showcase: build docker image and release it to the Container registry

[ge0ffrey]

The Timefold Container Registry ( https://github.com/orgs/TimefoldAI/packages ) should contain a docker image with the latest stable showcase so users can easily try it out.

That showcase container image should be able to run all the use-case quickstarts (preferably without having to install their container images).

[rsynek]

FYI @ge0ffrey Not yet finished and not yet public, but certainly possible:

#24

https://github.com/orgs/TimefoldAI/packages/container/package/timefold-solver-quickstarts-showcase

[rsynek]

Ideas to reduce the image size:
A) switch to alpine, explore jLink and sharing dependencies between individual QS
B) native compile all the QS and the showcase; make the showcase support the native QS

B will also make the startups much faster; no more waiting for a QS to become ready.