Using the default local Virtualbox Guest Additions ISO will result
in different builds depending on the system being used.
My Debian jessie system with Virtualbox 5.1.8-dfsg-6~bpo8+2
from jessie-backports/contrib is actually too old to build
stretch and newer as the kernel module can't be compiled.
So instead use a specific VBoxGuestAdditions ISO, currently
being the one recommended by upstream on https://www.virtualbox.org/
While at it:
* increase VM memory size from 512MB to 1GB during build, otherwise
we can't even use a temporary apt cache directory and will fail with OOM
* check for build errors in virtualbox logs
* /var/log/VBoxGuestAdditions.log doesn't necessarily exist,
so instead display the last 10 lines of the according log files
that actually exist