Because the LDAP DNS tests run in a container, they are targeted to one specific distro (Ubuntu 18.04). Since this distro does not run as part of a bang builder, we should extend test coverage to RHEL 8 builders.

This can be done by changing
1. jstests/external_auth/dns_container_tester.js to support RHEL 8
2. jstests/external_auth/lib/ldap_dns/Dockerfile - this file should be generated somehow per distro. Options include (1) generate the Dockerfile from a template via cpp and then build an image or (2) generate it and pipe it to docker build.