is possible to run two web apps at the same time, one using PHP the other using Ruby, each one on a Docker container ?
Docker is designed to run one software, if you want to run more than one, you need a tool like supervisor, s6, daemontools, check the doc for supervisor
Should be no problem. Normally you have one App per container. You could create a Docker container for your PHP server and a container for your Ruby server.
You need to choose different ports, because by default, both will run on port 80 or 443 and then it should work