After a searching the internet I found out that I need to add the line root:100000:65536 into /etc/subuid and /etc/subgid.
I think that line is added automatically when installing LXD but for some reason it wasn't generated.

@alan_g said in How to launch GUI apps from CLI?:

@nikalon you have to "trick" Unity8 into allowing the application to connect. I have not tried it recently, but something like this... https://lists.ubuntu.com/archives/mir-devel/2015-September/001267.html

Yes! That works, thank you! For anyone who wants the answer you have to append the paremeter --desktop_file_hint=unity8 or -- --desktop_file_hint=unity8 to a regular binary you want to open. This is the syntax:

./application -- --desktop_file_hint=unity8

Unfortunately the game crashes because of some OpenGL problem that I don't know how to solve yet:

library "/vendor/lib/egl/libGLESv2S3D_adreno.so" not found

----- Initializing OpenGL -----
Initializing OpenGL subsystem
Using 8 color bits, 24 depth, 8 stencil display
shutting down: Unable to initialize OpenGL (glAccum)
idRenderSystem::Shutdown()
Shutting down OpenGL subsystem
Sys_Error: Unable to initialize OpenGL (glAccum)
shutdown terminal support

But that's an unrelated question that I should ask elsewhere.

I'm following this guide for compiling Ubuntu Touch packages. Setting up LXD with crossbuilder setup-lxd works well but when I execute the command crossbuilder in any project I get this error:

error: no devices/emulators found
Could not find a device, will not deploy
Building address-book-app for armhf.
Creating LXD container address-book-app-usdk-16-04-amd64-armhf-dev using ubports-sdk:ubuntu-sdk-16.04-amd64-armhf-dev.
Error: Remote ubports-sdk exists as <https://sdk-images.ubports.com>
Creating address-book-app-usdk-16-04-amd64-armhf-dev
Error: Failed container creation: LXD doesn't have a uid/gid allocation. In this mode, only privileged containers are supported.

I tried removing and reinstalling LXD, removing my previous containers, rebooting but nothing works. I'm using Pop!_OS based on Ubuntu 18.04.