Add Praefect binary to Gitaly UBI container

What does this MR do?

This MR adds the missing praefect binary to the UBI-based image of Gitaly.

It also contains a few minor improvements.

Test

docker run --rm -t -i registry.gitlab.com/gitlab-org/build/cng/gitaly:2751-praefect-binary-ubi8 praefect -version

Related issues

Closes gitlab-org/charts/gitlab#2751 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated