Make sure file generated by docker-gen has the right permission

[buchdag]

#308 by @NicolasDorier, with a minor code formatting correction

If the destination file does not exists, the permissions become 600 because this is the default permission for temp files.

This is problematic, if the container consuming the destination file does not run as root.

Instead, the default permissions should be used (as per umask).
This PR will make permission 644, which is generally what we want. And if the container consuming it is not happy with it, it always can override the permissions.

[NicolasDorier]

Damn 2 years ago. I don't remember I even opened this PR, nor that I did write anything in the previous PR. :D