build is not hermetic

See https://github.com/splunk/splunk-operator/issues/1345 for similar request for the Operator. Pull base images based on digest so old builds are repeatable. 

https://github.com/splunk/docker-splunk/blob/9b55b6960ee00f54c19a25a86a178f998824711b/base/redhat-8/Dockerfile#L19