Hey @Imperol,
The encryption key location will depend on how you are running, It will be inside the container under /home/node/.n8n/config how you access this depends on how your options.
If you used docker run with a docker volume you can access the volume storage and pull it from there, If you use run with the storage being under ~/.n8n:/home/node/.n8n you can access it under the running users ~/.n8n path (this is also where it will be for npm installs).
Another option if using docker is to access the container then browse to the file so you could use docker exec -it [container_name] /bin/sh