feature: add .NET installation (#90)

* file: add .NET installation script dotnet.sh

* feat: add dotnet installation to custom image

* feat: add dotnet to build matrix

* Prepend `PATH` correctly

linux/ubuntu/scripts/dotnet.sh

@@ -0,0 +1,31 @@
+#!/bin/bash -e
+################################################################################
+##  File:  dotnet.sh
+##  Desc:  Installs dotnet CLI
+##         Must be run as non-root user after homebrew
+################################################################################
+
+# Install dotnet CLI
+. /imagegeneration/installers/helpers/etc-environment.sh
+
+printf "\n\t🐋 Installing .NET 🐋\t\n"
+#apt-get install -y dotnet-sdk-7.0 dotnet-sdk-6.0 dotnet-runtime-7.0 dotnet-runtime-6.0
+curl -LO https://dotnet.microsoft.com/download/dotnet/scripts/v1/dotnet-install.sh
+cat ./dotnet-install.sh
+bash ./dotnet-install.sh --install-dir /opt/hostedtoolcache/dotnet --no-path --channel STS  # net 7.0
+bash ./dotnet-install.sh --install-dir /opt/hostedtoolcache/dotnet --no-path --channel LTS  # net 6.0
+rm ./dotnet-install.sh
+export DOTNET_ROOT=/opt/hostedtoolcache/dotnet
+export PATH=$PATH:$DOTNET_ROOT
+{
+  echo "DOTNET_ROOT=${DOTNET_ROOT}"
+} | tee -a /etc/environment
+
+prependEtcEnvironmentPath "${DOTNET_ROOT}"
+
+which dotnet
+dotnet --version
+dotnet --info
+#dotnet --list-sdks
+#dotnet --list-runtimes
+printf "\n\t🐋 Installed .NET 🐋\t\n"