How a container works is that the file system has a series of layers. Like stacked sheets of glass with drawings on them. What you see at the top is the file

How a container works is that the file system has a series of layers. Like stacked sheets of glass with drawings on them. What you see at the top is the file system that results from what is on each sheet of glass. Things on higher sheets of glass can obscure or shadow things from lower layers. Your layer, the top most layer, might have only your JAR. Assembling and deploying your container could be very fast -- but docker takes care of that. You could organize your container so that you have a layer with your Tomcat. Another layer with your JVM. Then you can easily swap in / out layers to define different versions of your container. Whoever deploys your container on a server doesn't know or care what technology you use. Java, Python, etc. They just plug in the container and it runs. (Docker assembles its layers.)