Increase concurrent users by limiting container resources with docker-swarm
In general: No you can't. At least not on the Docker level.
You can maybe code your app in a way that it monitors such metrics and adjusts itself but Docker isn't able to.
What Docker CAN do is limit the app to 50% and kill it if it uses more than that by utilizing resource limits:
docs.docker.com - containers/resource_constraints
docs.docker.com - docker-compose resource limits