build-vyos: add hint about lo interface sysctl knob

(cherry picked from commit b0497191f7db7a5a37448ed7ba1f74321d0e2abc)

1 files changed, 7 insertions, 0 deletions

diff --git a/docs/contributing/build-vyos.rst b/docs/contributing/build-vyos.rst
index 792c4208..935f73b5 100644
--- a/docs/contributing/build-vyos.rst
+++ b/docs/contributing/build-vyos.rst
@@ -129,6 +129,13 @@ per release train (`current` or `crux`) - container. Add the following to your
 Now you are prepared with two new aliases ``vybld`` and ``vybld_crux`` to spawn
 your development containers in your current working directory.
 
+.. note:: Some VyOS packages (namely vyos-1x) come with build-time tests which
+   verify some of the internal library calls that they work as expected. Those
+   tests are carried out through the Python Unittest module. If you wan't to
+   build the ``vyos-1x`` package (which is our main development package) you need
+   to start your Docker container using the following argument:
+   ``--sysctl net.ipv6.conf.lo.disable_ipv6=0``, otherwise those tests will fail.
+
 .. _build_native:
 
 Native Build