Update contributor guide regarding consolidated Dockerfile templates

The recent refactoring to the Dockerfile templates (https://github.com/dotnet/dotnet-docker/pull/3404, https://github.com/dotnet/dotnet-docker/pull/3422, https://github.com/dotnet/dotnet-docker/pull/3445) are a significant change from the way things were done before. This should be documented in the contributor guide so people can understand how this all works.