This isn't a request for shipping container graphics, I actually have some to offer to you - finished ones.
I've tested them as drop-in replacements as-is without any scripting changes, and they nearly work - the edges trim off and the top edge also trims off (they're slightly over-tall so that loaders and inserts look more like they are correctly "behind" them, but that's a matter of personal taste to me) so they probably need some layering adjustments, and there's a subtle shading seam I need to fix up, but I have the graphics and also the blender source files ready to go if you have any interest in them.
Naturally I would probably just send the graphics to you, but I can provide a new "warehouse.png", and also roof decorations as separate sprites if you want them to be placeable anywhere on the roof of a container - you probably don't want to change your graphics layering code too extensively so I don't know about how feasible that is but the offer's there.
They're styled to match Factorio's steel chest, with some very (and I do mean very) small inspirations from AAI's look. I can provide links here if they're not filtered or I can post the pictures to your git page.