Implement custom script in smtp

[f-necas]

Custom scripts

This PR adds the capability to run scripts at startup of the container.

It's a part of the georchestra PR :

• Implement custom scripts georchestra/georchestra#4083

Soving issue :

• Add ability to launch custom scripts before the start of the app georchestra/georchestra#4030

How-to

Set CUSTOM_SCRIPTS_DIRECTORY environment variable to point to a valid volume.
Set a volume with scripts to execute.

Example :

 smtp:
    ...
    volumes:
      - georchestra_datadir:/etc/georchestra
    environment:
      - CUSTOM_SCRIPTS_DIRECTORY=/etc/georchestra/smtp/scripts

[edevosc2c]

There is a need to upgrade to FROM debian:buster, otherwise the container does not build.
Could you upgrade to this new image?