Change base build image to alpine

What does this MR do?

securego/gosec is built on Alpine which is based on musl libc. Things can go wrong if you mix binaries with glibc.

What are the relevant issue numbers?

gitlab-org/gitlab#216781 (closed)

Does this MR meet the acceptance criteria?

This just updates the build process, so I guess nothing is needed here.

• 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