docker: Don't use a hardcoded root password
Best practice is to use unprivileged service daemons inside Docker
containers. But with this hardcoded root password, in the case of
remote code execution, an attacker could trivially escalate their
privileges to root/uid 0. And while that's uid 0 inside a container,
that's a much larger attack surface.
Instead, do the same thing we're doing for the Cloud images: lock the
root password, create a user to make Anaconda happy, then delete the
user in %post.
https://bugzilla.redhat.com/show_bug.cgi?id=1175997