Add dotnet 6.0 image

This is to include a dotnet 6.0 image in addition to the 3.1 and 5.0 ones (although just thinking about it now it might have made more sense to just move the 5.0 image to 6.0 since 6.0 will be supported long term and 5.0 isn't).

I set up docker on WSL on my workstation to build this locally, all seemed to work; let me know if I missed anything.

This is in preparation for moving the skeleton to .NET 6.0, and then moving all of our .NET Core 3.1 projects to .NET 6.0.

dotnet-build-server/6.0/Dockerfile

-FROM mcr.microsoft.com/dotnet/sdk:5.0-buster-slim
+FROM mcr.microsoft.com/dotnet/sdk:6.0
 
 ARG BUILD_DATE
 ARG VCS_REF
 ARG VERSION
 LABEL org.label-schema.build-date=$BUILD_DATE \
-      org.label-schema.name="dotnet SDK 5.0" \
-      org.label-schema.description="Docker image to build .NET core 5.0 applications" \
+      org.label-schema.name="dotnet SDK 6.0" \
+      org.label-schema.description="Docker image to build .NET core 6.0 applications" \
       org.label-schema.vcs-ref=$VCS_REF \
       org.label-schema.vendor="University of Nebraska - Lincoln" \
       org.label-schema.version="0.1.2" \