Ahyuck.
The phases of the moon are not caused by the Earth's shadow. A Lunar Eclipse is. They are fundamentally different.
In fact , during a lunar eclipse, the Earth's shadow IS curved, gif related is the September 2016 Blood Moon eclipse , taken with my own equipment.
Pic related is a frame from the gif, overexposed so the Earth's penumbral shadow is more distinct.
Clearly it's curved.