I was having the same problem, and figured out a solution.
The codec needs to be updated inside the docker. Here are the steps I took to accomplish this:
1. ssh into your Unraid server (I am on Windows and use Putty)
2. Run the following command to be able to make changes inside the Air Video HD container:
docker exec -it <container_name> bash
(<container_name> should match what you see in the Unraid GUI, my was "AirVideoHD" so my command was "docker exec -it AirVideoHD bash" without the quotes)
3. You should now be making changes inside the container - the h.265 codec plugin is not available from the Ubuntu repositories, so you will need to add a repository:
sudo apt-add-repository ppa:strukturag/libde265
4. Update the packages from the newly added repository:
sudo apt-get update
5. Install the h.265 codec plugin for VLC:
sudo apt-get install vlc-plugin-libde265
6. Type "exit" to return to the Unraid command line (No longer making changes to the Air Video HD container)
7. Restart AirVideoHD docker container from Unraid GUI (may not be necessary, but I wanted to make sure the changes stuck after the container was restarted)
I was successfully able to stream and download videos using the h.265 codec and no longer received the "missing codec" error. (Air Video Converted the files to download to iOS)
I am not sure if this container is still being maintained, as it has been over a year since the last update, but if it is, then hopefully this can be included in the next update.
Hope this helps anyone having problems playing h.265 video files!