Docker refactor

What does this MR do?

Refactor Dockerfile to give is control over the base image that we use. This is so that we can quickly update the image if there are security vulnerabilities found by container scanning.

What are the relevant issue numbers?

gitlab-org/gitlab#299585 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer