docker: Run container with its own network namespace

docker: Run container with its own network namespace

For normal operation, there is no need for the container to run
in the host network environment or to have access to the users
X Session.

To accomodate usecases which need this, there is now a `TOPOTEST_OPTIONS`
environment variable to provide additional options to the docker run
command.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>