This is the repository which contains the Dockerfiles and supporting scripts for:
https://registry.hub.docker.com/u/gocd/gocd-dev/
https://registry.hub.docker.com/u/gocd/gocd-agent/
https://registry.hub.docker.com/u/gocd/gocd-server/
https://registry.hub.docker.com/u/gocd/gocd-build-installer/
Follow those URLs for more details about the actual images. For instructions to build the Docker images yourself, check the first line of each Dockerfile.