Dockerfile: Bump to f31

We should match coreos/fedora-coreos-config#200
on general principle, but also specifically because we now also have
a buildroot container that derives from this, and we want to support
people building content in that buildroot that targets the host.

Dockerfile

@@ -1,4 +1,4 @@
-FROM registry.fedoraproject.org/fedora:30
+FROM registry.fedoraproject.org/fedora:31
 WORKDIR /root/containerbuild
 
 # We split into multiple steps here so that local dev workflows which involve