The shape of the container is important because pressure waves refract and reflect of the walls of the container and this increases the distance and time the pressure waves need to travel. This phenomenon should be familiar to anyone who has experienced the imperfect acoustics of a poorly designed concert hall. All fluids are compressible. However, under some range of conditions, it is often possible to make the approximation that a fluid is incompressible. Water, for example, only changes its volume very slightly under extreme pressure.
Gases are much more compressible.
The compressibility of air, for example, is part of our common experience. If we assume a pump diameter of 1.25 inch, then a force of 18.1 lbf will be required. This is not a large force, so that raising the pressure by two atmospheres above ambient pressure is easily done. We call pressures relative to zero pressure absolute pressure. This is the pressure that appears in the perfect gas law.