ci: Drop tier-x build, do build test case

As part of all of this we're de-emphasizing "tier-x" and focusing
on making it ergonomic to either build up from minimal, or down+up
from standard.

Second, also add a CI test for our derived image.

Signed-off-by: Colin Walters <walters@verbum.org>

tests/Containerfile.test-derive

@@ -1,7 +1,7 @@
 # This test case exercises using the fedora-bootc image as a builder
 # to generate a minimal target image derived from CentOS Stream 10 content,
 # and then further extends it in a secondary phase.
-FROM quay.io/centos/centos:stream10 as repos
+FROM quay.io/fedora/fedora-bootc:rawhide as repos
 
 # This is intentionally a locally built image
 FROM localhost/fedora-bootc as builder
@@ -12,16 +12,15 @@ FROM scratch
 COPY --from=builder /target-rootfs/ /
 RUN <<EORUN
 set -xeuo pipefail
-# Verify we have CentOS content
 . /usr/lib/os-release
-test "$ID" = centos
+test "$ID" = fedora
 
 # And install a package
 dnf -y install strace
 dnf clean all
 
 # Cleanup and lint
-rm /var/log /var/cache/* /var/lib/dnf
+rm -rf /var/log /var/cache/* /var/lib/dnf
 bootc container lint
 EORUN
 LABEL containers.bootc 1