buildah's Makefile makes an unfortunate assumption about the loaded SELinux policy and tries to relabel the buildah binary with a type that doesn't exist in Gentoo's policy (container_runtime_exec_t vs. podman_exec_t). It seems setting SELINUXOPT to an empty value successfully tricks the build system to skip its SELinux relabeling step, then Portage will take care of it like normal.
Created attachment 887957 [details] build.log