ensure new container images are downloaded

***In GitLab by @terceiro on Aug 17, 2021, 15:05 GMT+1:***

Now, after a container image is downloaded, it will never be refreshed again. we don't want to hit the network on every run, but we do need to update the image from time to time.

*Migrated from GitLab: https://gitlab.com/Linaro/tuxrun/-/issues/25*

Start by locating the command-line entry point and the code that downloads and caches container images. Trace how a cached image is selected on repeated runs, then run the existing test or command flow that exercises image downloads. Done means cached images are reused without a network request every time, while still being refreshed periodically.

Written by the indexing model from the issue text.