ensure volumes are not deleted

[ruslandoga]

This is no longer relevant since Docker Compose v2 doesn't delete volumes on docker compose down but the previous behaviour should be documented just to be safe.

Relevant:

• plausible/docs#406
• https://forums.docker.com/t/why-docker-compose-down-deletes-my-volume-how-to-define-volume-as-external/67433/2
• https://stackoverflow.com/questions/65799945/why-docker-compose-down-deletes-my-volume-how-to-avoid-this-action-done-by-dow