Stainless steel is safe to use, but it's good to consider the container as a whole, not just the material it's made of. I can't tell if the bottom is a separate piece. If it is, I'd want to know this container is made in such a way that it will be absolutely leak proof.
Also a lye container should have a cover to control contamination (dust, flying insects, animal hair, etc.), to minimize the spill hazard, and to keep the lye solution from reacting with the open air. I keep lye solution covered as much as possible when I'm not actively pouring or otherwise working with it.