Add a base image reference for FIPS builds

What does this MR do and why?

Adds the current base image being used for FIPS enabled GitLab builds

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

doc/development/fips_compliance.md

@@ -474,3 +474,9 @@ Ubuntu 20.04 FIPS:
 1. Trigger `e2e:package-and-test` job, if not already triggered.
 1. On the `gitlab-omnibus-mirror` child pipeline, manually trigger `Trigger:package:fips`.
 1. When the package job is complete, manually trigger the `RAT:FIPS` job.
+
+### Base images for FIPS Builds
+
+Current: [UBI 8.9 Minimal](https://gitlab.com/gitlab-org/build/CNG/-/blob/master/ci_files/variables.yml?ref_type=heads#L4)
+
+Under Evaluation: [UBI 9](https://gitlab.com/gitlab-org/build/CNG/-/merge_requests/1460)