Use Debian Buster slim as a base image

What does this MR do?

Change the Docker base image to python:3-slim (Debian Buster slim), and install packages to better comply with the manylinux2010 policy.

What are the relevant issue numbers?

See gitlab-org/gitlab#196833 (closed)

MR initially created in the context of gitlab-org/gitlab#13694 (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
• [-] Conforms to the code review guidelines
• [-] Conforms to the Go guidelines
• [-] Security reports checked/validated by reviewer