fix some problems with Debian virtual machine

virtualbox-guest-* packages, currently available only on
backports repositories, are needed to auto-resize guest
display in VirtualBox and make Guest Additions work.

policykit-1-gnome allows to launch, as normal user,
applications which require root privileges (such as
Synaptic).

mkosi/debian/mkosi.postinst

@@ -52,3 +52,11 @@ ldconfig
 dpkg --add-architecture i386
 apt update
 apt install -y libc6:i386
+
+# Add backports repositories to install packages needed by VirtualBox (but it
+# might be useful also to install more recent software). It should be
+# safe, as packages from backports have low priority (100) and cannot be
+# installed accidentally.
+echo "deb http://ftp.debian.org/debian stretch-backports main contrib" >> /etc/apt/sources.list
+apt update
+apt -t stretch-backports install -y virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11

mkosi/debian/mkosi.vm.d/01-mkosi.vm

@@ -15,7 +15,9 @@ Packages =
     firefox-esr
     geany
     lightdm
+    linux-headers-amd64
     network-manager
+    policykit-1-gnome
     spice-vdagent
     synaptic
     tango-icon-theme