Dear Jeremias,
Thank you for your suggestions.
I’ve managed to switch the project from GTK to the FrameBuffer/DRM project. The application does indeed work better than earlier but now I’m facing a different problem. In the previous project the video player worked correctly but now when I’m in DRM mode I get errors. I guess it probably is due to the lack of the compositor? Can it be fixed or is there a guide on how to set it?
EDIT: It’s actually in FrameBuffer mode, I’ve mistaken the debug env variable with the docker one. DRM actually doesn’t work at all in my case 
ALSA lib confmisc.c:855:(parse_card) cannot find card ‘0’
ALSA lib conf.c:5180:(_snd_config_evaluate) function snd_func_card_inum returned error: No such file or directory
ALSA lib confmisc.c:422:(snd_func_concat) error evaluating strings
ALSA lib conf.c:5180:(_snd_config_evaluate) function snd_func_concat returned error: No such file or directory
ALSA lib confmisc.c:1334:(snd_func_refer) error evaluating name
ALSA lib conf.c:5180:(_snd_config_evaluate) function snd_func_refer returned error: No such file or directory
ALSA lib conf.c:5703:(snd_config_expand) Evaluate error: No such file or directory
ALSA lib pcm.c:2666:(snd_pcm_open_noupdate) Unknown PCM default
[000000559b7701c0] alsa audio output error: cannot open ALSA device “default”: No such file or directory
[000000559b7701c0] main audio output error: Audio output failed
[000000559b7701c0] main audio output error: The audio device “default” could not be used:
No such file or directory.
[000000559b7701c0] main audio output error: module not functional
[0000007e881b4b90] main decoder error: failed to create audio output
[0000007e60001410] gl vout display error: parent window not available
[0000007e60001410] gles2 vout display error: parent window not available
[0000007e60001410] xcb vout display error: window not available
Best Regards,
Daniel