[docs] Explain what hardened images mean

[cjellick]

When we say we've hardened the images, we mean the following:

• We are not simply mirroring the image from upstream. We are building on top of a hardened and minimalized base image, which is currently UBI.
• For any binaries written in Go, we are recompiling them using ouf FIPS 140-2 compliant build process (outlined here: https://docs.rke2.io/security/fips_support/#use-of-fips-compatible-go-compiler)

You can know if an image has been hardened in this way if it has hardened in its name. We publish image lists with our releases. Here is a sample one: https://github.com/rancher/rke2/releases/download/v1.22.3-rc1%2Brke2r1/rke2-images-all.linux-amd64.txt

We

Caveats:

• Currently, hardened images are multi-architecture. Only the Linux AMD64 architecture is FIPS compliant. Windows and the soon-to-come s390x architectures are not FIPS compliant.

[cjellick]

@jtravee I think this could go into an FAQ or maybe a new section in the security section.