You could try from a different browser or device just in case there are some cached credentials hanging around.
I don't know about the Docker part, but if you can find the command that launches the tvheadend binary within the container, add --noacl to the end of that command.
This will give you access to TVH without credentials. This will allow you to reset your password.
Once complete, remove --noacl, restart and then try to login again.