RHCOS is a derivative of both RHEL and Fedora CoreOS. The tool to build both RHCOS and FCOS is coreos-assembler. See the upstream documentation there first, and pull down the container.
coreos-assembler (or "cosa") accepts a configuration git repository as input. This repository is that configuration for RHCOS, just like fedora-coreos-config is for FCOS.
For example, you would use
$ cosa init https://github.com/openshift/os
to start. However, currently you need to configure the rpm-md repositories manually:
The RHEL repositories are only available to Red Hat customers, and it's likely that you want to use a mirror. And further, a common scenario is to inject specific versioned rpm-md repositories to test different snapshots or "composes". For these reasons, you need to provide the repos of input RPMs.
To do so, create a rhcos.repo file that looks like this:
# RHEL repos
[rhel-8-baseos]
baseurl=<url>
[rhel-8-appstream]
baseurl=<url>
[rhel-8-fast-datapath]
baseurl=<url>
# These are the OpenShift RPMs, see https://mirror.openshift.com/pub/openshift-v4/dependencies/rpms/
# except there's things like afterburn that are only internal right now unfortunately.
[rhel-8-server-ose]
baseurl=<url>
The names of the repos must match those in manifest.yaml.
The "api.ci" (CI cluster used by OpenShift builds itself) has a service that pulls internal RHEL repos: See https://github.com/openshift/release/blob/master/core-services/release-controller/README.md#rpm-mirrors
Use this:
$ cosa init https://github.com/openshift/os
$ curl -L http://base-4-8-rhel8.ocp.svc.cluster.local > src/config/ocp.repo
As noted above, RHCOS uses FCOS as an upstream. This is implemented by inheriting from the fedora-coreos-config repository as a git submodule; the RHCOS manifests and overlays include parts (though not all) of FCOS.
After a change is landed in FCOS, it's often OK to also immediately update RHCOS' main branch to the latest FCOS. We try to keep things compatible.
$ (cd fedora-coreos-config && git fetch origin && git reset --hard origin/testing-devel)
$ git commit -a -m 'Update FCOS'