Create a Dockerfile that contains multiple images

I have Dockerfile which builds an image for a Java project. I get this one up and running but I also need ExpressVPN or NordVPN installed in the same container.

I have a Dockerfile for NordVPN but I do not understand/know how to concatenate these two Dockerfiles into one so I only get one image to run.
(When I run the image I am passing in some environment parameters to the Java project thru JAVA_OPT)

I will provide my Dockerfile and info of NordVPN and ExpressVPN..