Templates for Dockerfiles

[mthalman]

Now that dotnet/dotnet-docker#1933 has the necessary infrastructure in place, defining templates for the .NET Fx Dockerfiles can be considered. There should be an investigation to determine where there is commonality between the Dockerfiles and whether its reasonable to define templates. Benefits of having templates, or at least going through the process, include being able to identify potential inconsistencies that were unintentional.