improvement: general changes

[Luc-cpl]

General changes to repo

1. Use docker volumes for better performance and docker organization (https://docs.docker.com/storage/volumes/)
2. Change caesar_site network to use internal docker compose network as caesar_supervisor already run as external network and is attached to Wordpress container
3. Change the docker-compose restart policy to unless-stopped instead aways
4. Remove Wordpress bind mount from current dir to use a volume instead, this increases docker performance and improve current dir legibility
5. Change plugins, themes ant mu-plugins dir to current dir as we do not need wp-content directory
6. Remove akismet and hello-dolly plugins from default install
7. Add the option to use a custom docker-compose.override.yaml file with the default docker-compose.yaml file, this allows the user to add modifications to docker structure directly (https://docs.docker.com/compose/extends/)
8. Set wordpress container user as host user to avoid problems when updating or installing plugins and themes via wp-admin