Re-enable RHEL CI (including offline)

[dleeapho]

We are using CentOS CI as a proxy for RHEL but we should get the infra setup for RHEL 7.5.

To be license compliant (while using Docker) we need a RHEL 7.5 host to build the image, to push to a private registry, and a RHEL 7.5 host to pull and run the image.

---

• Provision VSTS CI with RHEL 7.5: https://github.com/dotnet/core-eng/issues/3908
  • Docker, Git installed
• Set up VSTS to pull the image from ACR and run legs with the YAML build.
• Ensure some notification is set up so we know when the VSTS CI breaks. (This isn't GitHub PR CI!)
• Docker image:
  • Get broken VSTS RHEL 7.5 7.4 agent back up to build Dockerfiles.
    • Git PATH, figure out nested Docker: https://github.com/dotnet/core-eng/issues/3906
    • Red Hat subscription: https://github.com/dotnet/core-eng/issues/3921
  • Add RHEL 7.5-based Dockerfile in https://github.com/dotnet/dotnet-buildtools-prereqs-docker
  • Build the image using image-builder and push to ACR

Once this is set up, the VSTS RHEL7.5 CI can also be considered the official build.

[dleeapho]

/cc @MattGal per your triage question.

[dleeapho]

Related to 1. above: https://github.com/dotnet/core-eng/issues/3116

[dleeapho]

Investigating prereqs Dockerfile #520

[dagood]

This is now blocked on getting a sufficient number of up to date agents in the right VSTS instance: https://github.com/dotnet/core-eng/issues/3908 (FR).

[dagood]

We no longer plan to test on Red Hat, the value doesn't seem to be worth the infra implementation and ongoing maintenance. See #677.