Debian Stretch: install virtual box from virtualbox.org repository

These are steps to install virtualbox from virtualbox.org repository:

1. Add virtualbox.org repository into system:
   # echo 'deb http://download.virtualbox.org/virtualbox/debian stretch contrib' > /etc/apt/sources.list.d/virtualbox.list
2. Download and install virtualbox.org key
   # wget https://www.virtualbox.org/download/oracle_vbox_2016.asc
   # apt-key add oracle_vbox_2016.asc
3. Update your system
   # apt-get update
4. Install latest virtualbox (currently version 5.2)
   # apt-get install virtualbox-5.2

References:

• https://wiki.debian.org/VirtualBox