Docker restart on reboot:
So search the tvheadend docker container config
find /var/lib/docker/containers -name hostconfig.json -exec grep -il tvheadend {} \;
Edit the file
vi $(find /var/lib/docker/containers -name hostconfig.json -exec grep -il tvheadend {} \;)
Press / and type Restart to jump to the RestartPolicy.
If it is this:
"RestartPolicy":{"Name":"no","MaximumRetryCount":0}
Press i to enter edit mode and change it to this
"RestartPolicy":{"Name":"on-failure","MaximumRetryCount":3}
Then press ESC and type :wq to save and quit.