Update Dockerfile to include git

What does this MR do?

Adds git to the alpine docker image, because it is used in the Semgrep source code.

Semgrep uses git in two ways:

1. to determine unique project hashes (git ls-remote --get-url)
2. to crawl files and ignore those in the .gitignore (git ls)

What are the relevant issue numbers?

n/a

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