Dockerfile: default to Fedora 39 instead of 38

The action uses an external service to figure out what the current
version is, but as long as we have hardwired default, might as well keep
it up to date.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Dockerfile

@@ -1,4 +1,4 @@
-ARG FEDORARELEASE=38
+ARG FEDORARELEASE=39
 FROM registry.fedoraproject.org/fedora:${FEDORARELEASE}
 RUN dnf -y distro-sync && dnf -y install --setopt install_weak_deps=0 qemu-user-static /usr/bin/mount /usr/bin/chcon && dnf clean all
 COPY LICENSE entrypoint.sh /