[fix] buildprocess of docs in LXC (containers)

The $DOCS_BUILD variable (like all BUILD variables) in the lxc environment::

   sudo -H ./utils/lxc.sh cmd searx-archlinux make docs

is different from running build process in the HOST::

   make docs

with kernel-include directive we can use the environment variables in the reST
documents to address the correct file location of the include.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>