So that E2E tests in Github Actions can connect to Dimail container.
Previously we were attempting to connect as if from the outside. But
the E2E process is in fact inside the Docker Compose network.
("The tests came from inside the house !")
https://tvtropes.org/pmwiki/pmwiki.php/Main/TheCallsAreComingFromInsideTheHouse
this commit adds a script and 'make' command to populate dimail database
with basic objects: an admin account, a regie account, a domain
and an owner for this domain.
