Kde-pim docker not working. qt.qpa.xcb: could not connect to display :0

I am on Nixos 21.11. and tried to setup a KDE development environment using the kde-pim docker, following this tutorial
https://community.kde.org/KDE_PIM/Docker

I successfully installed the docker (at least it seemed so, I have an intel graphics card, so according to the documentation no extra step should be needed). But then starting kdevelop or qtcreator (both preinstalled in the docker), they both crash with Aborted (core dumped).

neon@b1df127876b0:~$ kdevelop
No protocol specified
qt.qpa.xcb: could not connect to display :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, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

Aborted (core dumped)

neon@b1df127876b0:~$ qtcreator
No protocol specified
qt.qpa.xcb: could not connect to display :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, wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, xcb.

Aborted (core dumped)

Anyone any clue how to get the KDE development environment working?
(I just would like to create a patch for dolphin, but failing at this pre-stage)

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