Rework CI Docker to use UV and fix ResNet18 test

[Victor-Jung]

Added

• A UV lock file to pin the package version.
• A Python version file.

Changed

• The Docker container we use to run CI now uses uv to prepare the dependencies in the uv cache.
• The ResNet18 test now tests the label prediction instead of the logit values.
• The readme is updated to contain the information on how to setup and run tests with uv.

[viv-eth]

LGTM and the tests are passing. I only had a minor nitpick and once this is resolved, we can land this PR.

[viv-eth]

Minor question; do we really want the -s flag in the ModelTests run? It is useful for debugging, but in the CI it would just create huge logs.. Unless there is a good reason to do so.