Merge pull request #11 from gnadaban/add-dotnet-3.1

Add .NET SDK 3.1

Dockerfile

@@ -33,11 +33,12 @@ RUN apt-get update \
 && apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF \
 && apt-add-repository 'deb https://download.mono-project.com/repo/ubuntu stable-focal main' \
 && apt install -y --no-install-recommends mono-complete=6.12.\* \
-# Install .NET 5
+# Install .NET 5 and 3.1
 && curl -fsSLo packages-microsoft-prod.deb https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb \
 && dpkg -i packages-microsoft-prod.deb \
 && apt-get update \
 && apt-get install -y dotnet-sdk-5.0 \
+&& apt-get install -y dotnet-sdk-3.1 \
 && rm packages-microsoft-prod.deb \
 # Install Chrome
 && curl -LO https://dl.google.com/linux/direct/google-chrome-stable_current_amd64.deb \