Dockerfile: update to Fedora 39

See: coreos/fedora-coreos-tracker#1490

Dockerfile

@@ -1,10 +1,10 @@
-FROM registry.fedoraproject.org/fedora:38 AS builder
+FROM registry.fedoraproject.org/fedora:39 AS builder
 RUN dnf install -y golang git-core
 RUN mkdir /butane
 COPY . /butane
 WORKDIR /butane
 RUN ./build_for_container
 
-FROM registry.fedoraproject.org/fedora-minimal:38
+FROM registry.fedoraproject.org/fedora-minimal:39
 COPY --from=builder /butane/bin/container/butane /usr/local/bin/butane
 ENTRYPOINT ["/usr/local/bin/butane"]