Nice man, but I have a question in mind what is different between your solution and activemerchant from Shopify other than containerized things?
It is not end solution. It does not provide API, backend UI, integration with a storage. It takes a bit more time to setup it. Plus, you have to be a developer (preferably Ruby one) to use it.
PayumServer requires as few programming knowledge as possible. You can use it with any of your app, written on whatever language you like.
It is already used in many projects, in production, well tested. The 1.0.0 version was released not long ago.