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>