I think it owes a huge debt, in almost every way - story, the idea of space truckers / grungy spaceships to O'Bannon's previous movie - Dark Star. If anything Alien is Dark Star but done with an awful lot more technical accomplishment.
Giger most definitely brought something new to the screen both with the Alien and the look of the space jockey's ship.
It also owes a debt to the films like
1. It the Terror from Beyond Space
2. Planet of the Vampires