On a fresh Manjaro XFCE installation I didn’t need all those tips from this thread. What I did:
- pamac install inetutils docker
- reboot
- systemctl enable docker
- systemctl start docker
- pamac install chromium-docker
- reboot
And everything is fine, now I can watch Netflix on the second display (an old TV). Sound via HDMI works out of the box, I didn’t test bluetooth or other outputs (my Pi400 only has bluetooth and HDMI).
UPDATE: A minor drawback seems to be, that docker sends audio signals only to HDMI-1. Otherwise it works quite well.