Attend to mykaul's comments from #4244 (Move Manager containers to be UBI based)

[mykaul]

Multiple nitpicks and not so nitpicks around the Dockerfile.

1. If it's available without auth., use Red Hat's registry for UBI images, nothing else.
   hint:

ykaul@ykaul:~/github/scylla-cluster-tests$ podman pull ubi9
Resolved "ubi9" as an alias (/etc/containers/registries.conf.d/000-shortnames.conf)
Trying to pull registry.access.redhat.com/ubi9:latest...
Getting image source signatures
Checking if image destination supports signatures
Copying blob 3d1c86608284 done   | 
Copying blob de21b519acd7 [=====>--------------------------------] 13.0MiB / 84.4MiB | 33.5 MiB/s

2. Make sure you don't add extra layers, if you don't squash the image later.
3. Cleanup of minidnf is via microdnf clean all

Hope this is all.

[mykaul]

Ref #4244 (comment)