Move gosec download into first Docker build stage

What does this MR do?

Move gosec download into first Docker build stage

Why:

• Downloading gosec with ADD is convenient but caches the tarball so that it can't be removed in a later Docker layer.

This change addresses the need by:

• Move the gosec tarball download, validation, and untar into build stage.

Side effects:

• A slightly smaller Docker image is created.

What are the relevant issue numbers?

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