Remove dotnet v3.1 container.

dotnet-build-server/3.1/Dockerfile

-FROM mcr.microsoft.com/dotnet/sdk:3.1-buster
-
-ARG BUILD_DATE
-ARG VCS_REF
-ARG VERSION
-LABEL org.label-schema.build-date=$BUILD_DATE \
-      org.label-schema.name="dotnet SDK 3.1" \
-      org.label-schema.description="Docker image to build .NET core 3.1 applications" \
-      org.label-schema.vcs-ref=$VCS_REF \
-      org.label-schema.vendor="University of Nebraska - Lincoln" \
-      org.label-schema.version="0.1.2" \
-      org.label-schema.schema-version="1.0" \
-      maintainer="Roger Feese <rfeese@nebraska.edu>"
-
-# Make sure that tools we need are installed
-# Add external nodejs LTS apt source
-RUN curl -fsSL https://deb.nodesource.com/setup_14.x | bash -
-# JRE, NodeJS for sonarqube scanner post-scan
-RUN apt-get update && apt-get install -y \
-    git \
-    openjdk-11-jre \
-    nodejs \
-    && rm -rf /var/lib/apt/lists/*
-
-# sonarscanner (https://docs.sonarqube.org/latest/analysis/scan/sonarscanner-for-msbuild/)
-RUN dotnet tool install --global dotnet-sonarscanner
-RUN dotnet tool install --global trx2junit
-ENV PATH=/root/.dotnet/tools:$PATH