How to know if the HW acceleration is working

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As far as I know, you need to both pass in the /dev/dri/renderD12* device into the container and ensure the user running Jellyfin (UID 1000 in your case) has access to the render/video group on the host. Group name differs by distro but I don't think passing in the card as you have will work. Either send the entire /dev/dri directory or go with the render device.