Make use of CIs as Maven Repository in order to use it for building Docker images without including dependencies
Docker images are after all VM templates, i.e. they have to be more or less self-contained: you get the image, you run the environment with all the dependencies.

To approach the challenge operationally, Docker supports reuse of environments through the FROM statement i.e. you could maintain a base image and on top a much smaller image with the app itself. 

But, from the strategic point of view, I would say your architects might want consider splitting the whole thing into smaller microservices - that would allow for 5 smaller images. Of course I cannot judge whether it's really possible.