Find & Pick an opensource project that provides same or similar features to shopify. Learn the method of how to self host which ever solution is picked. Be it regular web hosting or some sort of docker hosting.
Some of these projects also provide a hosting service. They just setup a server with the project preconfigured & ready for use. At that point you are on your own on how use the web app, they will only handle the hosting side of things. This might be a good option as long as its a good hosting service.