Use reusable containers

[bsideup]

Hey @glaforge! Excited to have you review my PRs again 😊
(Context: @glaforge welcomed me to the Groovy community ~10 years ago)

Testcontainers allows containers to be reused between the sessions, enabling much faster iterations (e.g. after this change, CallGemma runs in 5s on Testcontainers Cloud, vs 13s without it).

The proposed change shows how to use it. No pressure if you want to keep the image-based approach, nothing is wrong with it, just it is slower because, each time you run the test, it will start the container, even if the model is already pre-pulled.

[glaforge]

@bsideup by the way, I'm curious when this reusable feature will become generally available?