update container base images to Fedora 31

Also update the uid management in the frontend image to be more consistent with
best practices, as demonstrated by:

https://github.com/RHsyseng/container-rhel-examples/tree/master/starter-arbitrary-uid

This allows the dependency on nss_wrapper to be dropped.

docker/Dockerfile-tests-py3

@@ -1,4 +1,4 @@
-FROM fedora:29
+FROM fedora:31
 
 WORKDIR /build
 RUN dnf -y install \