Running a Qt GUI in a docker container

Question

So, I have a C++ GUI based on Qt5 which I want to run from inside a Docker container.

When I try to start it with

docker run --rm -it my_image

this results in the error output

qt.qpa.xcb: could not connect to display localhost:10.0
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

So I searched for how to do this. I found GUI Qt Application in a docker container, and based on that called it with

QT_GRAPHICSSYSTEM="native" docker run -it -e DISPLAY=$DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix my_image

which resulted in the same error.

Then I found Can you run GUI applications in a Docker container?.
The accepted answer in there seems to be specific to certain applications such as Firefox?
Scrolling further down I got a solution that tells me to set X11UseLocalhost no in sshd_config and then call it like

docker run -v $HOME:/hosthome:ro -e XAUTHORITY=/hosthome/.Xauthority -e DISPLAY=$(echo $DISPLAY | sed "s/^.*:/$(hostname -i):/") my_image

this produces a slight variation of the error above:

qt.qpa.xcb: could not connect to display 127.0.1.1:13.0
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Following another answer, I added ENV DISPLAY :0 to my Dockerfile and called it with

xhost +
XSOCK=/tmp/.X11-unix/X0
docker run -v $XSOCK:$XSOCK my_image

This time, the first line of my error was qt.qpa.xcb: could not connect to display :0.

Then I tried another answer, added

RUN export uid=0 gid=0 && \
    mkdir -p /home/developer && \
    echo "developer:x:${uid}:${gid}:Developer,,,:/home/developer:/bin/bash" >> /etc/passwd && \
    echo "developer:x:${uid}:" >> /etc/group && \
    mkdir /etc/sudoers.d/ && \
    echo "developer ALL=(ALL) NOPASSWD: ALL" > /etc/sudoers.d/developer && \
    chmod 0440 /etc/sudoers.d/developer && \
    chown ${uid}:${gid} -R /home/developer

to my Dockerfile and called docker run -ti --rm -e DISPLAY=$DISPLAY -v /tmp/.X11-unix:/tmp/.X11-unix my_image, again same error.

I also tried several of the ways described in http://wiki.ros.org/docker/Tutorials/GUI, same error.

---

Am I doing something wrong? Note that I'm working on a remote machine via SSH, with X11 forwarding turned on of course (and the application works just fine outside of Docker). Also note that what I write is a client-server-application, and the server part which needs no GUI elements but shares most of the source code works just fine from it's container.

I hope for a solution that doesn't require me to change the system as the reason I use Docker in the first place is for users of my application to get it running without much hassle.

Answer 1

You have multiple errors that are covering each other. First of all, make sure you have the correct libraries installed. If your docker image is debian-based, it usually looks like a line

RUN apt-get update && \
    apt-get install -y libqt5gui5 && \
    rm -rf /var/lib/apt/lists/*
ENV QT_DEBUG_PLUGINS=1

Note the environment variable QT_DEBUG_PLUGINS. This will make the output much more helpful and cite any missing libraries. In the now very verbose output, look for something like this:

Cannot load library /usr/local/lib/python3.9/site-packages/PySide2/Qt/plugins/platforms/libqxcb.so: (libxcb-icccm.so.4: cannot open shared object file: No such file or directory)

The bolded part will be the missing library file; you can find the package it's in with your distribution's package manager (e.g. dpkg -S libxcb-iccm.so.4 on debian).

Next, start docker like this (can be one line, separated for clarity):

docker run \
  -e "DISPLAY=$DISPLAY" \
  -v "$HOME/.Xauthority:/root/.Xauthority:ro" \
  --network host \
   YOUR_APP_HERE

Make sure to replace /root with the guest user's HOME directory.

Advanced graphics (e.g. games, 3D modelling etc.) may also require mounting of /dev with -v /dev:/dev. Note that this will give the guest root access to your hard disks though, so you may want to copy/recreate the devices in a more fine-grained way.