This is an adaptation from MS helper to install dependencies, but to install dotnet 3.1 only

setup_dotnet.sh

#!/bin/bash
set -e

# Select a default .NET version if one is not specified
if [ -z "$DOTNET_VERSION" ]; then
  DOTNET_VERSION=3.1.300
fi

# Install .NET as the sudo (non-root) user
sudo -i -u $SUDO_USER bash << EOF
curl -sSL https://dot.net/v1/dotnet-install.sh | bash /dev/stdin -c Current -v $DOTNET_VERSION
EOF