If journalctl -u <service>.service and not giving you the journal logs you want from your container, you can run machinectl -l to find the container's UUID and then run a journalctl -M $UUID on the container uuid to see logs.
~# machinectl -l
MACHINE CLASS SERVICE OS VERSION ADDRESSES
rkt-6d427a1c-6961-45a2-a055-721edddb8558 container rkt - - -
~# journalctl -M rkt-6d427a1c-6961-45a2-a055-721edddb8558
If your systemd service starting your docker container is not listed under machinectl list, then add the following to your systemd service file that is starting your container:
[Service]
Slice=machine.slice
Cheers!