Run setup on "empty but in git" folders

There is no way in git to add an empty folder, so a normal (looks like the default way), of doing it is to add an empty .gitkeep file.
Without an empty folder in your git project, tools like `fig` will fail mounting data.

Alternative 2 is to check for the existence of files like `postgresql.conf`, or even create a check-file only for this docker script.