Use multistage docker build

What does this MR do?

Switch Dockerfile to multistage build.

I don't believe this warrants a changelog entry since we are not modifying the behavior, only the build process.

What are the relevant issue numbers?

gitlab-org/gitlab#9736 (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