install dotnet core 3.1 into dev container

.devcontainer/Dockerfile

@@ -22,3 +22,12 @@ RUN apt-get update \
     # Install Docker Compose
     && curl -sSL "https://github.com/docker/compose/releases/download/1.24.0/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose \
     && chmod +x /usr/local/bin/docker-compose \
+    #
+    # Install dotnet core 3.1
+    && cd /tmp \
+    && wget https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb -O packages-microsoft-prod.deb \
+    && dpkg -i packages-microsoft-prod.deb \
+    && apt-get update \
+    && apt-get install -y apt-transport-https \
+    && apt-get update \
+    && apt-get install -y dotnet-sdk-3.1 \