VirtualBox guest modules are missing in Tails 2.10~rc1

This is a regressions since Tails 2.9.1.

From tails-i386-2.10~rc1.iso.buildlog:

[...]
Loading new virtualbox-guest-5.1.8 DKMS files...
First Installation: checking all kernels...
Building only for 4.8.0-0.bpo.2-amd64
Module build for the currently running kernel was skipped since the
kernel source for this kernel does not seem to be installed.
Compiling and installing a custom udev-watchdog program
[...]

Note that it is not even mentioning the -686 kernel (whose headers package is installed, so it would work).

Even stranger, this does not seem to affect Jenkins: https://jenkins.tails.boum.org/job/build_Tails_ISO_testing/9/ . Could it be due to the new Vagrant basebox (#12081 (closed))? I used it when building Tails 2.10~rc1, obviously.

However, I can see that I built other branches using that basebox which did result in the modules being built, but they were all still on Linux 4.7, so perhaps it in combination with the upgrade to Linux 4.8 (#11886 (closed))? For the record, then the .buildlog looks like this:

[...]
Loading new virtualbox-guest-5.1.8 DKMS files...
First Installation: checking all kernels...
It is likely that 4.8.0-0.bpo.2-amd64 belongs to a chroot's host
Building initial module for 4.7.0-0.bpo.1-686
Done.

vboxguest:
[... success ...]

Note that it detected the builder VMs kernel (4.8.0-0.bpo.2-amd64). Perhaps there’s some bug when the builder VM has the same kernel as the target Tails build?

Feature Branch: bugfix/12139-virtualbox-dkms

Related issues

• Related to #12104 (closed)

Original created by @anonym on 12139 (Redmine)