Too many open files

[skopanev]

Hi! Thanks for the nice library that solving many things!

Describe the bug
In my gitlab runner when I have 150+ images it says "too many open files" (on local machine works ok).
I see the problem is that you try to optimize all the images simultaneously.

It would be awesome if you could add a setting to set the maximum number of images to be processed simultaneously.

To Reproduce
Use 1 CPU 8GB ram droplet with docker to optimize more than 150 images

Thanks!

[skopanev]

Some details.
There are 2 locations: opening config files, and then optimization itself.

[dc7290]

Looking at the error details, it seems that the error is occurring during the copying of the configuration file, rather than during image optimization.
Also, I'm concerned that this process is being performed repeatedly.