The systemd servicefile codium-server.service is executed at boot time.
It executes a script startup-codium-server.sh which calls docker run ...., the details of which are not shown here.
The docker container being run will fork, and the service can/does not keep track of it. That's why Type=oneshot is used in the service file.
Lingering has to be set up so that the user service can be run even when the user has not logged on:
loginctl enable-linger $USER
To check that linger setting
loginctl show-user $USER -p Linger
Setup and start the service
systemctl --user daemon-reload
systemctl --user enable codium-server.service
systemctl --user start codium-server.service