Allow propagating `PIP_INDEX_URL` and `PIP_EXTRA_INDEX_URL` to the docker container of the dependency checker

What does this MR do?

Propagate PIP_INDEX_URL and PIP_EXTRA_INDEX_URL to the Dependency Scanning analyzer gemnasium-python to allow for custom PyPI registries.

cc @fcatteau for https://gitlab.com/gitlab-org/gitlab-ee/issues/6375#note_114903502

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• New variables documented in Dependency Scanning documentation
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

• New variables are tested in a test project like python-pip

Security

N/A

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/12810