Enable creation of db files at start time

[tardich]

I have a need to get some db_files to be created automatically at start time (for example, I'm providing a file from a configmap named smtp_generic_maps that I,m enabling support for through RELAY_EXTRA_SETTINGS environment variable. But then, I'd need to have the corresponding db file to be created at container start time. One idea could be to list all the files that would require a .db file to be created in an environment variable so that the entrypoint.sh script could parse and run postmap on these.