Thank you, Shield Arc, I understand your advise. And indeed I'll take it into account for the future.
But please take a look at another picture attached. In fact the bucket edge should be welded from opposite (bottom) side first and, after turning it over, - from another side (inside). Distortion may occur within bottom side welding. As you may see, maybe the welding of stiffening strips 15x40 mm (yellow arrows) from underneath could do that "bad job" of edge distortion. Continuous lines (red arrows) are of welding together of 4 mm sheet and 20 mm thickness edge. It is welded with MIG by another guy, not by me with my MMA.
If you follow my method, you'll have a fighting chance. But sometimes things will warp no matter what you do while welding. That's when you heat shrink. Or use a hydraulic jack, and a strong back, to force it into shape.