Add conditional check for current user:group id on run file.

[sturple]

Hi @janeczku,
Thank you for creating this docker image.
One thing I found with a larger dropbox installation, is that when you go through the run file where you do the chown https://github.com/janeczku/docker-dropbox/blob/master/run#L28, it can take a long time to re-do this process, especially depending on the amount of files in your dropbox.
A solution would be to create a conditional statement to see if they are already set, before doing this process. ie $(stat -c "%u" /dbox) Also noticed that this pull request might do the same thing ea15ed1