HelixReproVMs DTL for runtime repro needs

[dotnet-bot]

Migrated from https://github.com/dotnet/core-eng/issues/13466

@lukas-lansky wrote:

We should find out whether the current HelixReproVMs DTL setup works for runtime as it is, or something more needs to be done. To reiterate, the intended usecase for the DTL is "my build/test is failing on a particular OSOB-provided Ubuntu 16.04 only and I need a VM that is matching the CI configuration as close as possible".

Ideally, people won't need to this in the future for acquiring native build dependencies as we are going to fix Native Toolset Acquisition. They are still going to need that to troubleshoot other problems related to the precise OS and its configuration being used.

We already started documenting that DTL as a part of other epic. We also made sure it actually provides current images recently. It very well might be the case that the only thing that needs to be done is to enhance that docs with "how to connect from home using the DDFun proxy" info.