Running tests against an open source project without forking it

I am already using CircleCI for my project. I would like to test it against one of it’s dependencies, an open source project on GitHub. I don’t want to fork the open source project because I want it to be as up-to-date as possible and my tests to fail if breaking changes are made. I need to set the env var IP address of the open source project in both places if it’s not so that my project can make test api calls to it. On my local machine, I use Docker for Mac so everything is on the default localhost ip, but that may be different in CircleCI. I clone the open source project and run docker-compose up and I run my project’s tests. Is this possible? If so, do you have any advice?