Inside a Docker Container: "Error: cannot open display: localhost:11.0" - docker

Question

I am trying to use programs with a graphical interface in a docker container over ssh.
Currently I am connected over ssh on an external machine where docker and the containers are running. On the host I can start programs like firefox which was displayed correctly. The connection is established with:
ssh -Y root#host
When I try the same in a docker container, with the firefox image (see below):
docker run -it --privileged --rm \
-e DISPLAY=$DISPLAY \
-v /tmp/.X11-unix:/tmp/.X11-unix \
-v /root/.Xauthority:/root/.Xauthority:rw \
firefox
I just get:
Error: cannot open display: localhost:11.0
I already tried to set xhost + on the host, but it is still not working.
The host runs Scientific Linux release 7.2 and the docker image is created with the
Dockerfile from http://fabiorehm.com/blog/2014/09/11/running-gui-apps-with-docker/:
FROM ubuntu:14.04
RUN apt-get update && apt-get install -y firefox
# Replace 1000 with your user / group id
RUN export uid=1000 gid=1000 && \
mkdir -p /home/developer && \
echo "developer:x:${uid}:${gid}:Developer,,,:/home/developer:/bin/bash" >> /etc/passwd && \
echo "developer:x:${uid}:" >> /etc/group && \
echo "developer ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/developer && \
chmod 0440 /etc/sudoers.d/developer && \
chown ${uid}:${gid} -R /home/developer
USER developer
ENV HOME /home/developer
CMD /usr/bin/firefox

Answer 1

Adding --net=host to docker run solved the problem.