Tomcat is a servlet container. Docker is also related to containers. Why are they both called "containers"?
What is the difference between Tomcat containers and Docker containers?
The term "container" here is only similar in the basic English definition of a "construct" that contains "something".
Apache Tomcat is a Java process that contains J2EE servlets and JavaServer Pages.
A Docker container is an operating system (OS) construct that contains a usable OS (as close as it can get) separate from the host OS (as separate as it can). Docker itself isn't really the container either, Docker manages the underlying OS to make it easier to run an image as a container.
So both the "construct" and the "something" are vastly different between Tomcat and Docker which makes the technical definitions vastly different too.