Add python to base image

What does this MR do?

Install dependencies required by node-gyp and introduces new variable DS_PYTHON_VERSION where users can switch between python 2 and 3

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/12296

Does this MR meet the acceptance criteria?

⚠ Please note that since default python version is 3 based on requirement we have to explicitly say we would like to use 2.7 otherwise it will fail since node-gyp is not working with python3 yet. ⚠

Tested the container in our test project with package that required to be compiled.

• gitlab-org/security-products/tests/js-npm!18 (closed) ==> uses python 3.6
• gitlab-org/security-products/tests/js-yarn!15 (closed) ==> uses python 2.7
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30519
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30528

• 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