docs: Add BASE_IMAGE build-arg for kayobe image build

A Rocky Linux 9 base image is required for Zed onwards.

Author: markgoddard

doc/source/operations/tempest.rst

@@ -101,7 +101,7 @@ Build a Kayobe automation image:
     git submodule update
     # If running on Ubuntu, the fact cache can confuse Kayobe in the Rocky-based container
     mv etc/kayobe/facts{,-old}
-    sudo DOCKER_BUILDKIT=1 docker build --file .automation/docker/kayobe/Dockerfile --tag kayobe:latest .
+    sudo DOCKER_BUILDKIT=1 docker build --build-arg BASE_IMAGE=rockylinux:9 --file .automation/docker/kayobe/Dockerfile --tag kayobe:latest .
 
 Configuration
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