The functional tests by default only runs against the latest image tag

functional_tests does not use the image from the current branch and only uses the latest tag.

Right now to use the image for your branch you have to manually update TAG the gitlab-ci.yml file for the functional_tests job. This works, but it's easy to forget and this must be done for every new branch one creates.

Instead, we should detect the CI environment and branch in the scripts/test.sh script and update the TAG variable accordingly. This will improve our branch workflow and remove manual steps for contributors.