Add option to set the `sftp-user-inventory` GID

[emericv]

I need that all directories must be created with a specific GID.

Actually, the sftp-user-inventory group is created with a default “user” GID :
https://github.com/emberstack/docker-sftp/blob/main/src/ES.SFTP/Security/UserManagementService.cs#L38

For the moment, I have overridden the compose entrypoint like this:

    entrypoint: |
      /bin/bash -c "
        grep -q '^sftp-user-inventory:' /etc/group \\
          || echo 'sftp-user-inventory:x:100:' >> /etc/group
        exec tini -- dotnet ES.SFTP.dll
      "

Nice to have: an option to set a specific GID for this group.

Otherwise: the directories must be created with the GID of the user specified in the configuration instead of the sftp-user-inventory GID...