Document current usage of Docker

[michellemcdaniel]

Migrated from https://github.com/dotnet/core-eng/issues/8373:

Since the Docker improvement epic https://github.com/dotnet/core-eng/issues/6708 has been logged some time ago and Docker is used already in our infrastructure, the first step is to map and document the current situation.

The goal of this task is to document the actual use of Docker in our infrastructure and check the if the existing documentation (such as Documentation/Project-Docs/docker-image-usage-improvements.md) is still up-to-date. The documentation can be done either in this issue, or in a dedicated document.