Switch the container registry used for deploying images from docker.io
to ghcr.io (github).

To start using ghcr.io, regenerate your docker-compose.yml file via setup.mailu.io.

For Mailu 1.9 users:
1. docker-compose down.
2. create the file .env in the same folder as docker-compose.yml with the following contents:
DOCKER_ORG=ghcr.io/mailu
3. docker-compose up -d