How to officially install latest Docker into offline RHEL 8 x86_64 machine?

Using the centos repo from docker.com should work on RHEL and rebuilds including CentOS. Mirror what you need locally. Although yes Docker should figure out what to do about Stream.

Prior to 8, CentOS Linux released after the same RHEL version. CentOS Stream flips this around to be testing the next RHEL updates. It is RHEL, but with earlier exposure to changes before they are generally available.

Presumably Docker will continue to provide EL rpms after the end of the year. Like every other CentOS user, they will need to decide to build on Stream or RHEL or something else. As a user, doesn't really matter as long as it has been tested to work with RHEL.

Likely the RHEL repo containing only s390x is because that architecture does not exist for CentOS. yum would ignore the additional architecture, but users might get confused.

As of RHEL 8 Red Hat does not package or support docker, they switched to a different container tools set including podman. Official in this context is limited to docker.com packages.