Add Ubuntu-Slim image definition (#13423)

Add ubuntu-slim image definition

images/ubuntu-slim/scripts/build/install-git.sh

@@ -0,0 +1,34 @@
+#!/bin/bash -e
+################################################################################
+##  File:  install-git.sh
+##  Desc:  Install Git and Git-FTP
+################################################################################
+
+# Source the helpers for use with the script
+source $HELPER_SCRIPTS/install.sh
+
+GIT_REPO="ppa:git-core/ppa"
+
+## Install git
+add-apt-repository $GIT_REPO -y
+apt-get update
+apt-get install git
+
+# Git version 2.35.2 introduces security fix that breaks action\checkout https://github.com/actions/checkout/issues/760
+cat <<EOF >> /etc/gitconfig
+[safe]
+        directory = *
+EOF
+
+# Install git-ftp
+apt-get install git-ftp
+
+# Remove source repo's
+add-apt-repository --remove $GIT_REPO
+
+# Document apt source repo's
+echo "git-core $GIT_REPO" >> $HELPER_SCRIPTS/apt-sources.txt
+
+# Add well-known SSH host keys to known_hosts
+ssh-keyscan -t rsa,ecdsa,ed25519 github.com >> /etc/ssh/ssh_known_hosts
+ssh-keyscan -t rsa ssh.dev.azure.com >> /etc/ssh/ssh_known_hosts