What are alternatives for Docker in Docker (dind) usage for Jenkins slave running in container?
To avoid relying on dind (out of building new images from a dockerfile which should be ok) I'd look around Jenkins's kubernetes plugin.

It obviously needs to bring to life a kubernetes cluster but allow to have a one time jenkins agent to run the test in a clean isolated environment, and then tear it off. The maintenance costs of the kubernetes cluster should not be greater than maintaining your Jenkins slaves. 

The bonus point is that you can version you slaves with specific environment and don't have to tweak around for multiples version of ruby/java/python and let the task choose the proper "environment" by specifying the agent image to use and have a ready to test environment without any tweak pre-test. 

I'd head toward kurbenetes as container scheduler because it is relatively lightweight to configure and maintain.