Document image tests
What does this MR do?
Document how to run image integration tests locally. Please note that https://gitlab.com/gitlab-org/security-products/analyzers/common/#analyzers-development already covers other kinds of tests.
What are the relevant issue numbers?
gitlab-org/gitlab#330093 (comment 603203151)
Does this MR meet the acceptance criteria?
Changelog entry added-
Documentation created/updated for GitLab EE, if necessary -
Documentation created/updated for this project, if necessary -
Documentation reviewed by technical writer or follow-up review issue created -
Tests added for this feature/bug -
Job definition updated, if necessary -
Conforms to the code review guidelines -
Conforms to the Go guidelines -
Security reports checked/validated by reviewer