Dockerfile: install dnf5-plugins for copr support on Fedora 43+

On Fedora 43 dnf is dnf5, and the copr subcommand is provided by the
dnf5-plugins package rather than dnf-plugins-core. Without it the
ephemeral CI job fails with:

  Unknown argument "copr" for command "dnf5".

Install dnf5-plugins with a fallback (|| true) so it's a no-op on
CentOS Stream 9 where that package doesn't exist.

Assisted-by: OpenCode (Claude Sonnet 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

Dockerfile

@@ -27,6 +27,8 @@ COPY --from=build /out/ /
 RUN <<EORUN
 set -xeuo pipefail
 dnf -y install dnf-plugins-core
+# dnf5 (used on Fedora 43+) needs the copr plugin installed separately
+dnf -y install dnf5-plugins 2>/dev/null || true
 dnf -y copr enable rhcontainerbot/bootc centos-stream-9-x86_64
 dnf -y install bootc
 dnf clean all