You can package Jenkinsfile Runner with a specific Jenkins image and turn that into a Docker image. This way, you can ensure people are running Jenkinsfile in the specific Jenkins environment, for example one that's identical to your production environment.
First, follow the preparation step in main README and create a directory full of plugins. Name it plugins and put it next to Dockerfile.
Then build the Jenkinsfile Runner image like this:
docker build -t jenkinsfile-runner:my-production-jenkins --build-arg JENKINS_VERSION=2.108 .
The optional JENKINS_VERSION specifies the version of Jenkins core.
Run the image by mounting the directory that contains Jenkinsfile into /workspace. For example,
docker run -v~/foo:/workspace jenkinsfile-runner:my-production-jenkins