Kibana 7.5.1 release by Elastic

The Dockerfile was kept as aligned as possible with our existing Dockerfiles [1].

Notable changes from the existing 7.5.1 release include:

• using registry.access.redhat.com instead of nexus_doc as BASE_REGISTRY
• adding missing dumb-init
• using the non OSS version
• still using the tar.gz distribution instead of rpm
• minor changes in the prebuild.sh script
• different healthcheck

[1] https://github.com/elastic/dockerfiles/blob/7.5/kibana/Dockerfile