Consider SECURITY_SOURCES variable while picking Rails repo for QA Docker build
So that QA image is built from security repo of GitLab. Without this, we see failures like https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/663282565
So that QA image is built from security repo of GitLab. Without this, we see failures like https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror/-/jobs/663282565