Fix the GitLab template: entrypoint and stage

[satsura]

The GitLab template had never been run against a real project. Migrating 54 repositories onto it surfaced two defects that broke every pipeline:

• Entrypoint. A GitLab runner starts the job through a shell, so it invokes the image's entrypoint with the shell's flags — with ENTRYPOINT ["cerberus"] the job dies instantly with cerberus: Unknown option '-c'. Clearing it (entrypoint: [""]) is the documented fix.
• Stage. The job sat in stage: test, which only exists if the project happens to define it. Repositories with their own stages: (build/deploy) failed at config parsing — the pipeline died before any job ran. .post exists in every pipeline regardless, which is exactly why the DefectDojo template it replaces lived there.

Verified on our GitLab: all 54 xpc/cg projects now produce a valid pipeline with a running scan job.