I know it has been a month, and this post is not constructive. I too have this problem. I am using a Synology NAS with hardware encoding (support for two streams). I also did the same thing, and confirmed my docker container has read / write access to the device (/dev/dri) folder and contained devices.
I created a custom flow to test if encoder support hardware encoding, and once I got everything configure right, it passed the test. So I then implemented my flow, and it does not appear to be hardware accelerated, as my CPU is over 50% used, and the encoding occurs at 1.9x rate, which does not seem right for the encoding. I too would like to know if it is using hardware encoding, or is using software encoding.
Did you ever solve you issue?