Headless mode does not work

It seems that headless mode does not work:

docker run -dit --name sheepit -e SHEEP_LOGIN=nnn -e SHEEP_PASSWORD=mmm -e SHEEP_NO_GPU=true -e SHEEP_MEMORY=3G -e SHEEP_HEADLESS=true -e SHEEP_CORES=2 -e SHEEP_COMPUTE_METHOD=CPU sheepitrenderfarm/client:text

This produces the following error message in the logs (docker logs -f containerid):

ERROR: Dec 22 17:40:14 Blender can not open a display to create an OpenGL context needed for EEVEE/GPencil. Will try another project in a few minutes. The error happened during the test frame render. Restart the client and try again.