Docker works in layers. Depending on how your layers are set up, it may be just a single diff from the existing image you already have, which can be quite fast.

Docker works in layers. Depending on how your layers are set up, it may be just a single diff from the existing image you already have, which can be quite fast. However, if you change one of the base layers, like Alpine Linux it will rebuild all layers above that. Good container design should make that rare.