Hide tests in GitLab CI for automatic grading


I would like to use GitLab CI for automatic grading. Students should get feedback (e.g., task 1 passed, error X on task 2), but they shouldn’t have access to the actual tests, because some of them would give away the solution. That is, they shouldn’t be part of the student’s repository.
Is this possible with GitLab CI? How would this be solved? The only way I can currently think of is putting the tests into the pre-built docker image.

Thanks in advance!