Use docker compose cp to copy database backups into the database container.

[megri]

I recently noticed when doing a divio project pull db <live|test> that having the project directory mounted at /app in the database container is required for the script to succeed (I had removed this line from my docker-compose.yml-file as it looked like it was redundant.)

Instead of expecting this directory to be present, the downloaded database backup could be copied directly to the running container with docker compose cp. This would likely be less brittle than the volume mount.