[kdomanski]

Not even that is necessary.

> The best solution probably is to create a new database during image creation, but in the official Postgres image this option is still not available.

The official image's README (https://registry.hub.docker.com/_/postgres/) says otherwise - both a user and a database will be created with name $POSTGRES_USER (if set).

Furthermore, one can run postgres in single-user mode to prepare any initial db image on top of the official one.