Build gemnasium-python images

What does this MR do?

• Add a child pipeline to build, test, and push the gemnasium-python images.
• Refactor the CI configs for child pipelines, and extract a shared CI template.
• Move the CI configs for the child pipelines to .gitlab/ci.
• Rename CI var BUILD to IMAGE_ALIAS.
• Rename build/main to build/gemnasium, to match the name of the image being built.
• Move the CLIs to a new cmd directory.

What are the relevant issue numbers?

gitlab-org/gitlab#198361 (closed)

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
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer